I've seen suspicious craigslist ads in the past.

Many bikes photographed in the same spot. Poor-quality photographs (blurry, etc.) that make it difficult to pick out distinguishing marks. Very little detail about the bike's components.

None of these are proof of a stolen bike, but each is a useful flag, an indicator that the ad is worth further scrutiny, and flagging could help people looking for their bike.

My thought was - recruit your friends to flag it with you. When things get stolen you normally tell your friends, who tend to be sympathetic. Better than no flagging, though I admit this can be gamed quite easily of not policed well (one set of users flagging several bikes could be of concern for example)