if the listing on ebay states clearly "sold as seen" and no money back/no return etc, then the guy cant return it and get money back as that is your contract with him same as if you buy summit from a shop online etc.
Forget the written receipt, well i guess its an added bonus, so keep it, but if your ad on ebay clearly states that when you buy you cant return the item then thats basically it, so dont worry about it.
Its like , if you went to a car showroom, and it says on the window of the car, caution ! if you buy this car it is non refundable or returnable, i guess they wouldnt get the car sold, lol, but at the same time i would imagine they would then have the right to refuse money back or return, just as you can do, what would be the point in even having adverts that state these things otherwise, know what i mean, would be a waste of time, i dunno just how the law go's though, but i think if its sold as seen then its sold as seen, simple as that.