Ebay selling my missing amplifier

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.

There is a chance that it is a legitimate sale. If the box was destroyed, it could have gone to claims. The damaged or "lost" goods get sold at auction and the new owner can do what they want with it. Don't approach the situation with an accusatory attitude. Good luck with this debacle. It's always extra risky sending things overseas. Keep us posted. 

I would definitely NOT contact the current seller.  If you do, you may never see the amp again as possession of stolen property is itself a crime.  I would work through the local police or eBay fraud.  As someone suggested, you can also buy it from the seller and then dispute the sale as stolen property.

Was the package insured and have you been made whole by the Carrier/Insurer?  Thats question #1

if you contact the seller telling him it's yours, you are giving him a warning essentially to pull it from ebay and cover his tracks. Use your knowledge to your advantage. You don’t catch a criminal by emailing him that you are onto him