Dan,
Are you sure the buyer isn't Peter Nash?
In all seriousness, it sounds like this amounts to a small percentage of your overall sales revenue. If that is the case, take the high road.
First, ask the buyer to provide front and back scans, along with proof of purchase. Provided that they follow through with this request, and you are confident that the card wasn't altered since leaving your hands, refund the money, block the buyer, and move on.
As a bonus, send it in for grading. If the buyer didn't alter the card, it should come back with a nice number grade, giving you the opportunity to make all of your money back...and then some.
Just my two cents.
Best regards,
Eric