First, report it stolen to the police. Provide proof of purchase with the serial number and screenshots of the eBay auction.
Message the seller - inform him that he has listed a stolen item and you want it back. Give him whatever you have to prove it's yours.
If the seller won't cooperate, eBay has a special service available only to law enforcement to recover stolen merchandise. The police detective handling your case has to follow up and there is a good chance you will recover your amplifier.