The owner chose them, did he not?
And so he has responsibility for his (supposed) loss. Doing their best isn't in the contract.
Now the question is whether the owner would like to know about it. I think not, unless he plans on making use of their service again.
On top of that, you say they didn't solicit a price from you before selling, and so you may have thought about the price more seriously, had they done so, and maybe this extra number of dollars wouldn't be there if there wasn't the bias of possible anger/pride (no offense). For all we know, the sale could have been a good sale, and that's why they didn't auction. They're the real-estate agents, and you're the someone who they didn't solicit. Sorry if that's a bit rude (I have testosterone problems), but that's the way it is. They're not malicious agents, they do what they can. I think it's better to let it be (but I'm also unsure).
(Also, if they had to chase every best offer then there would be no end to it. They're making trade-offs and maybe you valued the property differently than most people would value it. It's not so easy to say that they're being incompetent when we're incompetent in this domain ourselves.)
It shouldn't be the "importance to me", but the importance to everyone and everything. On top of that, dilemmas tend to be about what we have a bias in. The calculus of virtue is a real danger, and unwise. We shouldn't do it, but we do it anyway. Remember, the bright are the most likely to be biased.
The compromise is to at least ask opinion to another person.
"The young man knows the rules, but the old man knows the exceptions." - Oliver Wendell Holmes
I was questioning your judgment for the sake of argument, but you're probably right about the numbers. Without more knowledge of the area, it's impossible to say if you're being reasonable or not, and it doesn't really matter. You say it's not about yourself, but you wouldn't know it if it was about yourself, and that was what I was trying to say. It's not about you in particular, but about you being the prejudiced party. That's something to take into account in the resolution of the dilemma. But I should be more clear/careful.