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.
Ebay selling my missing amplifier
8 months ago I sent an amplifier to europe for upgrades. The amp went missing and there were investigations that went unresolved. I happened to be searching on ebay and noticed the amp and when I magnified it Bingo the serial number was same. It is an ebay seller store that is listing it. Ebay has not yet responded after several attempts. What next?