I can't find the sold items on your Ebay account, but, for most categories, Ebay define 'used' as: 'An item that has been previously used. The item may have some signs of cosmetic wear, but is fully operational and functions as intended.'
So, your buyer is right to expect a fully functioning item.
You will have to accept the return.
If you don't want to pay for return postage, then you can refund without the buyer sending the item back.
If you sell items that are not working properly, you should always list them for parts/not working.
"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)