You listed the item as 'used' which means it is fully functional, then added 'sold as seen, untested' to the description.
No items sold on Ebay are seen before purchase - so 'sold as seen' means nothing.
The buyer would have expected a working item.
If it's not what they expected and they have opened a 'not as described' case, you do have to pay for a return label and refund in full when you get the item back.
This is Ebay's definition of Used:
Used
An item that has been previously used. The item may have some signs of cosmetic wear, but is fully operational and functions as intended. This item may be a floor model or an item that has been returned to the seller after a period of use. See the seller’s listing for full details and description of any imperfections.
Saying an item is 'untested' does not mean the buyer can't return it. You should have a read of the Money Back Guarantee. It is important that sellers understand the rights buyers have,
"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)
