If the buyer paid through Ebay checkout and collected the item themselves, they are covered by the Ebay Money Back Guarantee,
Unfortunately, you are responsible for getting the item back. Of course, you could collect it yourself or arrange your own courier.
It does sound as if they are angling for a partial refund - that might be an easier solution in this case.
"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)