That's 100% your amp.
Take a couple of mates incase he gets shirty, get him to bring it to the door to look at, social distancing, you know, and just walk away with it.
If he claims he bought it fair and square then he's as bent as the one that stole it. You can't buy that amp and sell it on for a profit at that price without knowing somethings iffy about it, plus he hasn't even got anything to test it with so why would he have bought it?