What did you sell the buyer?
Was it a watch or something else?
If they open a return case, you should accept it and provide a return label.
Before you do this, message them to ask if they are sure they have got the right seller as you are certain that you did not sell them the item they want to return.
"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)