[vostrocity]

You have to consider how difficult it is for the police to do anything in that circumstance – it's just 'he say, she say.' It might be easier if you've filed a police report and your bike is registered, but even then there is no evidence (especially on the spot) that the seller (who is probably just lingering around and not an actual registered flea market seller) stole the bike. I think part of it is also the culture: you don't want cops coming into a flea market to arrest people, because the whole flea market is a bit of a legal grey zone.

However, you should definitely press charges every time if you do get the police involved. That's the only way to reduce bike theft. If you don't press charges, the thief simply goes and steals another bike the next day.