Even if you plan to go to Action Fraud, I would absolutely still advise any seller to accept the return, provide a label and get the item back.

Your son could have sent the buyer a different card - his case is, I think, quite hard to prove.  He has neither the card nor the payment. If he had accepted the return, he would at least, hopefully, have the card back to sell again.

Good luck with Action Fraud though.

 

All sellers should be fully aware of the Money Back Guarantee and its implications for sellers.

Ebay state:

 

I don't accept returns, why am I getting a request from the buyer?

Buyers can always submit a return request, even if your returns policy says you don't accept returns. Whether you need to accept the return depends on their reason for opening the return.

If the buyer is asking to return the item because it's damaged, faulty, or didn't match your listing description, then you need to accept the return. If they've opened the return for another reason, for instance if they ordered the wrong item or changed their mind, then you aren't required to accept the return – however, where possible we always suggest providing a great customer experience.

 

The onus is not on the buyer to prove the item is not as described.

 

https://www.ebay.co.uk/help/selling/managing-returns-refunds/handling-return-requests?id=4115

"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)