OP's OP didnt even add up:
"Friday evening, another evening browsing Craigslist."
"Sorry to the dude who's Pista I rode for a few years and sold; maybe you should have posted on craigslist/lafixed/bikeforums/stolenbikeregistry/internet."
Agree with all that said you should have walked away as soon as it seemed fishy and reported to police. Especially agree that you should have declined the reward. At best, let the gracious owner of the stolen bike buy you a beer and move on.