Ask to see the receipt if he had it put in by a pro he should have one. If it was me i would tell him to do one If it was in FULL WORKING order when you sold it then he has fucked it (we all know how easy it is done ) and now wants to see if he can get his money back you get no guarantee with second hand electrical goods unless stated in the auction i had someone try it with me and i know the item i sold was in FULL WORKING order when i had it so i told him he must of fucked it up when he fitted it and he came out with oh a pro fitted it i said lets see the receipt then and then i got oh i cant find it . If you know it was working when you had it dont refund him and he dont get the postage back even if you do refund him even paypal dont give you the postage back if you get a refund .