You're kind of boned since you screwed up and sold it.
1) I would have never accepted it back at all as it was him and your aunt, not you.
2) I would have given him a way to get in contact with her as many ways possible as she was being the problem.
3) He was a moron for not checking it out that well or asking for a service record if he paid that much as you said.
Honestly you only have one thing you can do at this rate and it's dealing with him only. You got two choices, get the busted piece of crap back if you can then dump it on him and let him deal with your aunt. Or two, you took responsibility for it after you sold it out from under both of them, so damn well pay him and put the deal to rest. Honestly knowing what I do from my college years you're culpable for it now entirely by law because you sold it and had no right to. Your aunt is out of the picture unless you can get the unit back. If you can't, you pay him because he could just turn around and sue you easy and win because of the sale you made. If you can get it back, pay whatever it takes within reason, and then drop it off with your aunt and tell him she has it and that you're no part of it. He can then harass the hell out of her and sue her ass for selling faulty goods and you're in the clear.