Get the police involved, they may already be aware of this individual and they may already be under investigation.
This individual may just be a one man crime wave, or a part of a large organised crime syndicate.
Or then again the seller might not be the person who actually stole your bike, but they may know who did, or they may be completely innocent (apart from handling stolen goods)
Whichever it is let the police know before you do anything at all.
There have been cases where a small incident like this has killed a huge investigation off (we have a lot of police using our bike shop & we do from time to time hear things from reputable sources)