My guess:

All right, suppose that all of us liked exactly the same objects exactly the same amount. This obliterates the poorly-written-textbook’s reason for “trade”.

This is nonsensical, because whether an individual person likes something is not a constant. If everyone liked oranges just as much as apples,, trade would still be possible because the 1st orange is preferable to the 99th apple and the 1st apple is preferable to the 99th orange.

people can share expertise via Matrix-style downloads which are free

Just because people can share their expertise for free doesn't mean that they will. If Alice has practiced her orange-picking skills more than Bob has, it makes sense for Bob to give Alice some fruit in exchange for Alice's superior expertise.

Curated. I enjoyed how this post was a little journey of deconfusion from the inside. It went through some of the actual cognitive motions one might make when trying to understand economics. (Or, rather, when trying become less confused about questions like "Why does everyone's lives today seem so much better than people I read about in history books?" or "How is it that the guy at Papa John's down the street can spend a few days making pizza, and then go to the store... and return with a little all-in-one pocket camera-computer-telephone-thing more powerf... (read more)

A few things that seem relevant (5 things, but maybe not crystallized in the right way to be 5 separate answers to the OP's question):

Quantity: mismatch between natural quantity produced and quantity desired. Maybe I can plant an apple tree & pick the apples, but I don't want a whole treeful of apples. Maybe the most efficient way to make t-shirts is to build a big machine that makes a million shirts, and a process for making just 10 of them is wildly inefficient in terms of resources per shirt. (related keywords: economies of scale, capital investment... (read more)

Even in the final hypothetical world, there are still enormous gains to be had from specialisation. Gains from specialisation arise because while a person may be imagined to have zero task-switching cost, most tasks still have non-zero person-switching cost in that hypothetical world. Specialisation necessitates trade, so that each person can get what they need but do not make from the people who make those things to trade for what they need. (One could talk about trade being unnecessary in a “perfectly planned” economy, but that would be going off in a di... (read more)

Looking through the comments, it seems like most of my thoughts have been captured (economy of scale, collaboration producing non-linear accumulation, etc) But some of the others (risk management, the time axis of logistics) helped me come up with a new one: perishability. When we combine some of the other factors (especially risk) people will at times have a perishable surplus. At these times they would seek to convert this surplus to something non-perishable or some other thing that they need at the time. If we had a society as described above plus uniform starting conditions and everyone used the same dice-rolls (i.e. everyone had the same good/bad corn year), I believe this reason for trade would cease to exist

.

One more magical power of trade, that I didn't see in other comments:

Planning and logistics. It takes about a week and a dozen steps to make a pencil. (Ok, probably not all dozen of those steps need human intervention, but some probably do.) That's not too bad; I can set a Calendar reminder to ping me when various steps are done so I can move the materials to the next one. But to use reminder software I need a laptop. How long and how many steps does that take to build? I would guess years of time and tens of thousands of steps. So even if I technically co... (read more)

Working for a salary is also fundamentally a form of trade. In my case, I trade my time and expertise in writing software, and my company pays me.

Purchasing items is also trade - I trade the money I earned to buy goods and services from others.

To completely eliminate trade, I think people would need to be able to magically wish anything they desire into existence. (If you've watched The Good Place, I'm imagining something like what Janet is capable of.)

At that point, scarcity no longer exists. And since the main point of economics is to solve problems of scarcity, economics doesn't have much purpose either.

In advance of other comments: 
1. Declining marginal utility of specific goods and non-uniform initial distributions of goods over people (this one matters).
2. There is a finite length of the production chain that one person can accomplish if something would take longer than an entire human lifetime to produce. Suppose I luck into a massive amount of unobtanium, with perfect property rights. To some extent this is 1, but I might also desire goods that I could not produce in an individual life, and by trade acquire them (this one isn't that critical).
3.... (read more)

Specialization. Yes. I've been making an induction puzzle game over the past couple of years. It takes place largely on the workbench of a reverseng (derivation: Reverse Engineer) employed by a drone factory that is situated in a late industrial society (a few years after creating misaligned AGI, so hardened by its state of biological warfare that it will last a few years yet).

One of the organizing principles that has allowed your world to run so terrifically fast is "Specialism", which holds that a person's profession should cast a long shadow over their ... (read more)

There's still the problem that two people can't occupy the same space at the same time, so we need people to be able to swap places instantly. This then requires some coordination, which is mentioned below.

Some commenters have mentioned economy of scale—It can be more efficient to pool together resources to make a bunch of one thing at a time. For example, people want paperclips but they could get them much faster if they operate a massive paperclip-making machine rather than everyone making their own individually. I think this is already covered though, a... (read more)

Intellectual property seems like another important thing. For example, a game like Cyberpunk 2077 takes man-centuries of work to produce, but once it has been created, producing additional copies for trade is cheap. In the no-trade world, everyone has to recreate all the IP for themselves.

First: I'm also thinking of 'getting paid for a job' as a trade, fundamentally it's 'generating value' for money. It's similar with most investment opportunities: they either provide (immediate) value or have the capability to generate value. This will stay at least until no one needs that anymore, maybe because everyone is being administered by an AGI and robots, so no one has to work.

Second: Sharing fruits of labor/economies of scale. Basically the physicalized version of 'expertise sharing', e.g. I'm going to build your house while you're out working, b... (read more)

I would put it like this:

1. Comparative advantage
2. Incentive to produce
3. Distributing differential value (could be better phrased)

The option to make exchanges that increase my total value, incentivizes me to use my comparative advantage to produce new things that will be low value to me, but high value to others, so I can trade them. This increases the total value in the economy by both creating new value and distributing it in a more beneficial manner.

If the textbooks you've read leave you needing to figure these things out on your own, may I recommend David Friedman's 'Price Theory: An Intermediate Text', readable for free on his website. (I don't know enough about econ to either endorse or anti-endorse the sections that argue against government intervention in markets but it does nicely explain his position on such things.)

A neat thought experiment! At the end of it all, you no longer need to exchange fruit, you can just keep the fruit in place and exchange the identity of the people instead.

This is sort of hand-waving away the problem of locality. Star Trek transporter or not, there is only a finite space on which certain things can be easily produced. And this production space is embedded in time as well as space.

So you must also add to this hypothetical world that the production space(s) must be large enough to accommodate each person's self-production at an arbitrary point in time (to avoid task switching/ allocation friction). I.e. the iron mine must be large enough to accommodate everyone on earth at the same time.

A spatial framing:

(1) All objects have positions in space
(2) The desire by people to consume and use objects is not uniform over space (cars are demanded in Los Angeles more than Antarctica)
(3) The productive capacity to create and improve objects is not uniform over space (it's easier to create iron ore from an Australian mine, or a car at a Detroit factory)
(4) Efficiently satisfying the distribution of desires over space by the distribution of productive capacity over space necessarily involves linking separate points in space through transportation of g... (read more)

As we all know, studies on separated identical twins show that happens every single time, with no exceptions.

points of trade

Information exchange.

Even if you have teleports, you don't know where things are. Distribution of resources still matters even with 'equivalent desires' - if a car needs iron and platinum and other things and I have iron and you have platinum...neither of us has a car. (If the teleports have zero energy cost, or involve perpetual motion machines the case for 'why trade' just changed dramatically.)

Sharing information easily doesn... (read more)

Can't we simplify and say that the point of trade is to alter the entire system so that the final state is of higher utility to both participants than the initial state?

You have 2 apples and I have 2 oranges, but we both prefer the state where we have 1 of each fruit.

We both work 1 hour in the mines and 1 hour on the fields, but we both prefer the state where the skillsets of individuals overlap less.

At the end of your list the point of trade still exists, it's just the list of viable trades has been reduced.

Sometimes, in trading, one accepts a fixed value and in exchange offers something in return of near infinite value to all of society until the end of time that others build upon.

The title kind of threw me a bit. At first I was thinking a long the lines of at the time of trade. Clearly a different direction.

I also find myself a bit in disagreement with pretty much all the responses and perhaps the point of the post. What everyone seems to be talking about is not what I would call the point of trade. I would call them all the forces towards trading, the enablers of trade opportunities or something along those lines.

This is not to say everything said doesn't have a place in a model. But they all seem one step away. Most of what was s... (read more)

there are two more magical powers

Zero size in 3rd dimension and time travel.

Them: The point of trade is that there are increasing marginal returns to production and diminishing marginal returns to consumption. We specialize in producing different goods, then trade to consume a diverse set of goods that maximizes utility.

Myself: Suppose there were no production possible, just some cosmic endowment of goods that are gradually consumed until everyone dies. Have we gotten rid of the point of trade?

Them: Well if people had different cosmic endowments then they would still trade to get a more balanced set to consume, due to diminishing ... (read more)

I really enjoyed this. Taking the time to lay this out feels more useful than just reading about it in a textbook lecture. The same way doing a math or code problem makes it stick in my head more. One of the biggest takeaways for me was realizing that it was possible to break economic principles down this far in a concrete way that felt graspable. I think this is a good demonstration of that kind of work. 

Some further magical powers of trade:

1 It seems to (at least under certain conditions) reduce the incentives of states to declare war, and thus reduces the massive wealth destruction that comes with inter-state war.

2 It allows new (productivity-enhancing) technologies and practices to spread faster.

3 It enlarges the potential market on which novel products may be sold, and thus improves the expected ROI from investments in technology/ product development.

4 Positive network externalities (e.g., the value I get from my personal smartphone increases as a function of the total number of worldwide smartphone users).

Many of those powers were about surpassing human difference, so I would guess towards more of that:

1. Radical transhumanism, free-shared-expertise style, to make physical properties of one's body irrelevant to the outcome
2. Free and riskless preference modification, so that when you need a hundred thousand chairs making them yourself is a perfectly enjoyable pastime.

Intertemporal arbitrage: buying corn when there's a bumper crop and selling it when there's a drought.  How do we get rid of that?   Either time travel or giving everyone lots of storage space and prior knowledge of all the goods he will ever need and their future abundance/scarcity time series.

Price signals arising from trade are also an incentive for consuming less of / producing more of scarce things to make them less scarce.  Without the incentives of prices we'd need some other way of enforcing rationing of the finite capacity of the iron mines and communicating each person's marginal utility for iron.  A borg hivemind.

Wouldn't the accumulation and sharing of expertise still be a form of trade?

My initial ideas (e.g. cases where time are important) are pretty well captured by other comments, but in reviewing my thoughts I noticed some assumptions I was making, which might themselves qualify as additional requirements to eradicate trade:

A) I assumed that the skill-download feature includes knowledge downloading and no task requires more 'knowledge+skills in active use at a time' than the human brain can feasibly handle. If this is violated, specialization is still somewhat valuable despite free and presumably-unrestricted knowledge-sharing.
If you ... (read more)

Going to go big here:

Trade has intrinsic worth.  If both parties benefit from the trade, or both perceive themselves to have benefitted, trade's purpose has been accomplished.  It's not about the things that are exchanged, but the people doing the exchanging.  People enjoy trading.

Trading is fun.  It makes you feel good when you make a "good" trade.  What I mean by this is a trade in which you know you have benefitted and so has the other person.  Or at least you know you didn't try to rip them off, which I think is an awful f... (read more)

Are we also presuming that you can acquire all desired things instantaneously?  Even in a situation when all agents are functionally identical, if it costs 1 unit of time per x units of a resource, wouldn't trade still be useful in acquiring more than x units of a resource in 1 unit of time?  Time seems to me the ultimate currency that still needs to be "traded" in this scenario.