After you've filed your stolen bike report, the best bet is to arrange with the seller to meet up, and arrange with the police to be there with you. Show the police the record of your serial number, and both you and the police can verify that it's your bike. The police can then simply inform the seller that you're recovering your stolen bike. You take the bike home. End of story.
I've found that the police are more than happy to show up to assist in resolving disputes, than they are in doing the work themselves to recover stolen property. This way, there's no need to involve district attorneys, or go through the legal system process. It saves everyone a bunch of trouble.