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, as if everyone has the same innate traits and preferences then they can all just operate the machine for one microsecond and collect their paperclip. This solves the coordination problem, as if everyone knows what they want and how much labor is required per person then they all just switch between each task accordingly. The only coordination is about who goes where and when.
So in the end, we have people indistinguishable by location, preferences, abilities, knowledge, and behaviour under any given local conditions. It seems that there is some sense in which the people in this world are 'indistinguishable' as agents (barring potentially unnecessary differences like eye colour). I think this hints at trade being unnecessary if and only if the agents are 'indistinguishable' in some sense.
Edit: We may also need need equivalent local conditions as well. If everyone wants a house within 3 miles of anyone else, and there is a single 3-mile-diameter circle that is the best spot in the whole world, then only one person could occupy that spot. If one person happens to have that spot, they could rent it out for a time in exchange for things they want. Boom, trade.
Or for a more obvious example, one person has all the military power. This person does no work and gets stuff in exchange for not killing anyone. It's extreme, but probably technically an edge-case form of trade.
I wonder if there's a different potential takeaway here than "find what feels rewarding". Duhig’s story makes me think of a perspective I've learned from TEAM-CBT: Bad habits (and behavioural patterns in general) are there for a reason, as a solution to some other problem. An important first step to changing your behaviour is to understand the reasons why not to change, and then really consider what is worth changing. It sounds to me that Duhig figured out what problem eating cookies was trying to solve.
At least, that's the theory as I understand it. I haven't put it into practice, so I'm just spitballing here. But my personal experience is that when I have tried to create habits by setting up an arbitrary reward (eg. eating a block of chocolate) it has not been effective at all for me.
I've learned to be resilient against AI distortions, but 'octagonal red stop sign' really got me. Which is ironic, you'd think that prompt would be particularly easy for the AI to handle. The other colours and shapes didn't have a strong effect, so I guess the level of familiarity makes a difference.
I think the level of nausea is a function of the amount of meaning that is being distorted, eg. distorted words, faces or food have a much stronger effect than warped clock faces or tables, for example. (I would also argue there is more meaning to the shape of a golf club than a clock face.)