I would just tell him if he thinks there is a problem to file a dispute with PayPal. And stop any further communication with him. Use this as a lesson for the future. The nature of this hobby stuff breaks people dont like to eat the costs of their mistakes. I for one can only support my hobby habit not someone else's so I take extra precautions now. Anytime you sell something film it. Film it working,flying, serial numbers and you disconnecting the power. It may seem like a hassle but it only take 5min to do and you never know what they buyer may claim. So if you film it in detail you will likely have video proof showing the one item they are complaining about. Also send the buyer an invoice instead of them just sending you money. You can create default invoice templates. At a minimum in the return policy put no returns.
When I get around to it I intend to put a better disclaimer in the return policy something along the line of "by paying this invoice you acknowledge there is no implied warranty and all goods are sold as is. There is no returns for any items even for claims of the item being defective"
