buyer/seller integrity question

I did not post/give an AS IS bill of sale & it was an out of state purchase so he went by my descrip/pics I posted. EDIT this is my 65 dart I just advertized on here. I had given ~18K for it. I posted 11K for it which I thought was fair. the guy wants $5500 refund saying it had too much rust & other issues that I did not disclose. I only drove the car twice & never inspected it/pretty much just lost interest in it right from the getgo (other issues came up). the guy gave me $9700 for it. If I owe him I will pay him. RR