I would try to outbid the other guy if it were a really high end bike, then report it (say you found it) to the police department assuming that someone would have reported it stolen and they would probably happily pay you back for the bum's crack money (I wouldn't pay more than like $150 no matter what the bike just because I wouldn't want to potentially lose the money). On the other hand if nobody reported it stolen it's yours to part out on ebay or keep