I think some of the issue is that the pre-flip price was published knowledge. Had we not known what he paid, and had he not been so flamboyant in posting it the next day in the same arena, I doubt anyone would care.
If I buy something from you I really don't care what you paid for it. But if you tell me you paid way less than what you sold it to me for, it's certainly not going to give me the warm-fuzzies about the transaction, especially if I'm paying retail.
You don't visit a store and they post the price they paid. That would just eventually piss you off.
This is a remarkably good point, in my opinion. Not only does it rub salt into the buyer's wound, but my guess is it also negatively impacts the new seller's opportunity to sell the bicycle so soon after purchase.