Unhappy buyer .What to do ???

Used parts to me are sold as is!
When I purchase a used part, I look at what I have to lose & compare it to a new piece.
No one twists my arm to buy a used part.
I do base it off the information provided, be it an ad or speaking with the seller.
Even then, its a gamble & you have to be ready to accept it as a life lesson if it fails or doesn't meet your expectation.
Judging by your ad, I would have bought it.
Giving the guy a refund is up to you, but the buyer would have to pay the shipping frt. back to you.
You would then need to inspect it for correctness, some people may try to cheat you.
I would only refund the purchase price & the buyer should feel fortunate to get any refund on a used part.