Ebay selling my missing amplifier

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

If you collected a insurance payment from the loss, it isn’t yours anymore, the amp belongs to the ins. Co.

if that is the case, just report it to them, they are the experts on recovering 

That's horrible, hope you get satisfaction.  Good luck!