Easy, put a mark on the back of the item with a UV pen (a small mark).
If someone returns an item to you, check for the mark. Simple.
Or take pictures of something on it (a particular scratch or dent)
but it doesn't prove anything and ebay wont listen
otherwise as a seller you could take a pic of a good serial, then sell a broken item when they send it back "oh but look mr ebay the serial is different they celarly are scamming me"