The buyer can't have opened a return request if they have not received anything.
Have they opened a case through Ebay stating the item has not been received?
They need to do this rather than just messaging you.
You always have to prove delivery, not just that you posted the item.
If it was the card you sold a few days ago (23rd January) they may receive it today.
If you sent the item using the Simple Delivery label provided by Ebay, and the buyer opens a case stating they have not received their item, then Ebay should deal with it.
So, has the buyer opened an 'item not received' case?
Did you send with Simple Delivery?
"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)
