Ebay return (not same condition)

Sold a monitor on ebay which had the wrong power cable, buyer requested a return which I accepted same day. Over a week later the return arrived and there is clear signs of use. The screen is filthy, marks all over it and lots and lots of hair can be found. Really disgusting if I do say. There are scratches on it now and even found a piece of a leaf inside.

I am not amused, buyer kept item for a week so he clearly had no issue using the item without the correct power lead.

How do you deal with this on ebay?