Offer to refund the purchase price (not the postage because you've paid that out) if he posts it back to you at his expense. He'll probably decide he can't be arsed because of how much it costs to send back compared to what he's paid already and you'll never hear from him again. But, at worst, if he does leave you negative feedback you can reply to it saying you offered a refund but he refused and it won't look bad on you. Bish bash bosh, job done.