I think after you lowballed him and he took offense, you should have left it alone and walked or made another realistic offer. Instead, you lectured him. If I were the seller, I would have said to {censored} off at that point.
Agreed.
Dude, just let it go. Even though you're not gonna want to believe this, you're the one who caused it to escalate.
He didn't want your amp. He didn't want your really {censored}ty offer. And YOUR attitude wasn't helping to smooth over the situation, either. You turned it into a lecture and came across as having an "I don't need you, you need ME, pal" attitude.
Get over yourself. You're both jerks.