Ebay sided them him. Is it that easy that someone can just say its damaged and send books?
It may also be possible that he is sending you a broken system, different from the one you sent.
I always make a habit of logging the serial numbers of any consoles I sell, and any other high price items get an inconspicuous mark with a UV pen.
Earlier this year, I almost got burned on a $40 DVD. Girl ordered it, I sent it and a few days later I get an email saying that she ordered the wrong one. She returns it and I only opened the case just to check to make sure the disc was in there. I relisted it and it sold a week later. Fortunately, I double check all of our discs before shipping and holy crap was this disc scratched to hell. Definitely not the DVD I mailed her.
She basically used me to swap out her damaged disc for a nice minty one, for the cost of shipping. Luckily, it was all surface damage, so I was able to buff almost all of the scratches off. Since then, I've been marking my expensive discs with a UV pen