Receiving stolen property, knowingly or not, is most likely a 2nd class felony in your state. I would go into this armed with whatever law you have on your side.

The "seller" had better be able to provide the police with the location of the "flea" market. Most flea market operators charge for the stalls and should have records of the "seller" and can track it back from there. If the guy that bought it at the flea market can not provide a sales receipt then maybe you can get the guy he bought it from for failure to collect an pay sales tax on the transaction at a minimum.