Unscrupulous ebay buyers

Beware if you sell on ebay. I’m probably stating something most already know, but anyhow, I listed a brand new cartridge there which was bought and shipped on time. Buyer receives It, opens it and installs it. He proceeds to play his records. He then messages me that the cartridge has muddy bass and poor channel separation. I advise him to check all alignment parameters, but he insists cartridge is defective, I highly doubt it. I do not accept returns as stated in the listing. He proceeds to pack it up, taking a photo, and states "on the way!"...I again told him that I do not accept returns for a cartridge that is now used...also said that just because you bought something you have never listened to and you are displeased with its sound or performance, does not give him the right to demand I accept a return. I reported him to ebay, as in my opinion he is pulling a fast one. He could very likely have damaged it while installing it. It was brand new pristine condition upon shipping it. I am standing my ground and will not accept a return. If it shows up at my door, too bad. At this point, I can care less if I receive negative feedback, it would be my first. Worse buyer I’ve encountered in 20 years of selling. Man I hate ebay. Honestly, to me it sounds like buyer remorse, not a defective cartridge, plus I am not the manufacturer....he can pound sand.