I'm sure someone may correct me, but as a private seller, you are not covered by the Distance Selling regulations, therefore it becomes 'caveat emptor' - buyer beware. You have no legal obligation to the buyer, and depending on how they paid, they could claim against their credit card, or through Paypal, but if it was a bank transfer or a cash sale, then there's no recourse, except the Small Claims Court (it may have now been rebranded)
It then comes down to you as a seller as to whether it was a bit of a pup that you tried to get rid of, or it was 100% when you sent it and someone is trying it on. You have to decide whether some negative feedback on here is possible or acceptable, or fight fire with fire.
Only you know what you sent out, so only you can really decide which way to play it