Not sure if the information is here in the posts but is it being sold in the US. The KC post is on to the action. A friend of mine had a bunch of watches stolen from his house burglary . Just like you he was on EBay and there were his watches. He contacted local law enforcement where they were listed and they set up a buy sting.
Even if the guy selling it didn’t steal it its now stolen not lost merchandise as its found and this person or organization is not the rightful owner. Depending on circumstances maybe they do not prosecute but you can get your amp back as long as you have all documentation showing you are the rightful owner.