It will depend on whether he opens the dispute on eBay or through Paypal.   I don't like your chances with eBay as they require proof of delivery and with no tracking you don't have that, so they would probably make you refund.  If he goes through Paypal, they only as for proof of postage, so if you still have the receipt form when you posted and it has the postcode on it, you may be okay.  I send a lot of things by letter, but I always photograph them at the PO counter when they stamp them.  It only takes a second and Paypal accepts it as proof of postage.