I have a question about TDT's application in 33 of HP:MoR.

I didn't say you should just automatically cooperate. Not on a true Prisoner's Dilemma like this one. What I said was that when you choose, you shouldn't think like you're choosing for just yourself, or like you're choosing for everyone. You should think like you're choosing for all the people who are similar enough to you that they'll probably do the same thing you do for the same reasons.

Should businessmen collude on one-shot pricing? The decision theory I learned in school says "Never!," but I can see Harry's beliefs going in either direction.

Links to a good summary of TDT are welcome.