I traded with a guy from out of my area. I was cleaning it up today and noticed the serial number and model number have been scratched off. Geez.
Well, this brings up the moral question-- What should you do about it? Then there is the practical question. What can you do about it? You don't know for sure it is stolen. If you think it is, where would you even report it or take it to find out? It probably came from out of your area. In that case, the local police probably don't have a report for it. Do you contact them and ask? I don't know what you paid for it. Let's assume $500. How much time/money might be spent by you or the police trying to research it? I'm not one for doing illegal stuff, but I think I would just keep it and not worry about it. If I felt guilty about having it, I would maybe check some online or music shop bulletin boards. If I felt really guilty about it, I would contact the police to see if they had a theft report.
I'd ask the person where he got it from and if it bothered me enough I'd ask would they be willing to sign a letter of ownership. Nothing fancy maybe something like... I John Doe traded xyz to Bob Smith for xyz. Keep it simple. A signed piece of scrap paper may/might make you feel better. With his contact info.
How long has that model of amp been in production? It's entirely possible that the previous owner (and maybe even the owner before him/her) bought it "legitimately".
You did nothing wrong, but may now be in possession of stolen goods. It sounds like you have no breadcrumbs back the amp's history, but I'd probably be inclined to contact your local authorities and get something on record with any information you can supply about who you got it from. Sure I'd feel bad for whoever got clipped and assuming it is stolen, my concern at this point would be: What if the original owner spots it and lays claim? A scratched off serial number would not go far in exonerating you. Documentation that you did what you could to rectify your accidental participation in crime. I'd certainly at least reach out to local LEOs for advice...