Offer to buy it. When you go to pick it up, check the numbers. If it's yours, tell the guy that it's a felony to steal something worth that much (this will probably catch him of guard). Then tell him the cops are on their way and he should call his lawyer to try to get 5 years probation as opposed to a year in jail. He'll probably leave but if you are worried about it just bring a few friends to jump out of the bushes and scare the bejeezus out of him.

If you get there and find that it's not your bike just find a reason why you "don't want it anymore" (ie. you didn't tell me about this dent!) or just tell him why you wasted an hour of his time.

I hate criminals