Hmm - technically you are not obligated, as long as you did not expressly state or imply any warranty; but then, imagine how you would feel as the purchaser. You both acted in good faith to acheive a win - win result; neither of you could have foreseen the failure. If avoiding negative feedback on Ebay is important to you, you may want to come to some kind of compromise. You could offer to split cost of repairs, I suppose. That's being a "nice guy" and I don't think I'd go beyond that, personally. Or, like the poster above said, you could assume it was shipping damage and do nothing (unless you insured the package when you shipped it, in which case file a claim with the shipper).