Buyer wants to return item that he sold and was returned to him.

[value1collectibles]

I sold a used car stereo. It worked perfectly when I sold it and listed it as such. Buyer then sells the stereo to a second party, but it doesn't function properly for him. Now the original buyer wants his money back. Does admitting selling the item negate the Money Back Guarantee? I am trying to work it out with the buyer. But since I know it worked when I sold it, I'm just trying to cover my basis in case it escalates to a case. I would appreciate any help. Thanks