You need to accept the return and provide a label.
Refund when you get the item back.
If you don't deal with this, Ebay will refund the buyer and tell them not to return the item.
I can see grubby marks on the boxes in the original listing so maybe the item was not as pristine as you thought.
"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)