If they paid with Paypal and you sent with appropriate tracking, RING Paypal and point out to them that you had seller protection. Be polite but firm. Get the name and employee number of whoever you talk to. If they won't help ask to speak to a supervisor. If they still won't give you your money back, inform them you will refer the matter to the FOS. If they are still recalcitrant, go to the FOS.

The point of Paypal seller protection is it protects the seller if they comply with the rules for protection. If the credit card use is fraudulant (unlikely - it is almost certain the buyer is fraudulent), that is Paypal's issue.

Also put the buyer on your blocked bidder list.