What happens if you buy a stolen motorcycle from a dealer?

You get charged with possession of a stolen vehicle I believe. If you knew it was stolen when you bought it then you are handling stolen goods (or a similarly fragrant nosegay of legalspeak, depending where you live) and the police will be keeping a close eye on you and they will be very interested in you. If you did not know it was stolen, then the motorcycle will be confiscated and searched so they can try and find out who had stolen it and where it came from. How you then get your money back, if at all, depends again on where you live.