It's annoying, but the answer is to cancel the order and refund, no point sending out the item , as it may well only come back under a case of ' item not as described', and damaged.
Cancel the order citing ' Buyer Requests to Cancel' this refunds the buyer, and your seller performance is not affected.
Block them from returning and messing you about again.
https://www.ebay.co.uk/bmgt/BuyerBlock?
@fuggin.ada