Contact the police, arrange an appointment with the seller (without telling him anything) and let one of the officers go there with you (in civilian clothing, of course). Also make sure you have a proof of purchase and/or the police report of the theft.
The seller may have bought it without knowing it was stolen, so don't accuse him right away.
I had a similar situation, and the seller managed to prove that he bought the bike more than half a year after it was stolen. I agreed to buy back my own bike at a very low price.
I also came across a bike that was stolen from a friend of mine, and because the theft was reported and I had the paperwork on my phone, the police cut the lock and let me take the bike home.