Has buyer opened a not as described case?
If not he is probably hoping you will refund or offer a discount, but you arecnot required to do anything unless a case opened.
If a case is opened then you have to provide a reply paid label, refund on return and then you can report abuse of the return system to ebay.
Failure to do this and if buyer escalates ebay will just refund at your expense without requiring the buyer returns the cards.
Beware buyer may return real fakes, again you can report to ebay, but as they do not see what is sent or received, or for that matter be able to identify fakes, do not expect anything to be done.
