Bummer for your situation. First thing is I would do is a google search in his state and see if he has sued others for similar problems.
Have an attorney send a letter to stop harassing you.
Change accounts, just in case.
Remind him he bought it after it was inspected twice already and your done with it. Anything that he finds or damaged while he uses it is on him. Also, your not responsible for damage after it leaves your possession from improper transport or driver repellence.
I would not even offer to take the car back. He could have damaged it or swapped out some valuable parts with cheap ones.

Personally I would send a pic of myself in a Las Vegas club surrounded by pretty girls, my crew, lots of bottles of house booze and a couple black bags on the table with a caption saying ‘I spent it all buddy. Go pound sand or get buried in it).