Whether the seller was the thief or not I'm sure the statute of limitations is up on the theft. The seller could be charged with possession of stolen property, but you (or local prosecutor) would have to show the seller knew or should have known the bike was stolen.

To the OP I wouldn't keep bidding on your bike. I would notify the seller that the bike is stolen and that you have contacted your local police for the report and you are will forward that report on to the seller's local police. Keep of copy of this transaction. I would also print out the ebay page and seller information. Once you advise him the bike is stolen youi don't know what he is going to do, but if he should sell it then you have information to prove that he knew it was stolen and had possession of it.