Flipping is essentially the bicycle equivalent of a used car salesman. Nothing wrong with the practice, but how it's done can give it a bad name, as with cars.

I've seen people get really peeved because the bike that sold for $40 on yesterday's Craigslist is now on there for $400. I don't see anything wrong with that, assuming it is worth the $400. Seeing items that are overpriced is irritating. If the advertising is deceptive in some way, that's just wrong, but that's not a flipper issue, either. Just reading the Craigslist ads, it's not apparent who is a flipper and who isn't, but it doesn't appear that overpricing is limited to flippers, and they may be less likely to overprice, for that matter. They're selling old bikes, not heirlooms.