I wanted to sell my xbox 360 on trademe (to get funds towards a PS3 slim).  It's the original pro version circa 2006 (20GB Hard drive, no HDMI port). It's never had RRoD though, so that is good.

I figured I might get $200 for it with a bit of luck, so I set the reserve at $100 and offered a buy now of $300.

Less than one day after listing a buyer took the buy now option and sent me an email with their address (They are in Invercargil, I'm in auckland) saying they have deposited the money, along with postage of $15, direct to my account.
All good so far, although I think they are slightly crazy for paying that amount for this when you can buy a newer 360 for less than that, and often with games bundled in.  Heck, even EB games sell them second hand for about that price.

Then I check their history. Perfect feedback (107, all positive) is a plus, but I notice the last two auctions they won in the last couple of weeks were for a 360 with a broken DVD drive and an Xbox 360 DVD drive, together for only around $150.
I figure, ok, maybe they buy and resell 360s or something, but then none of their previous auctions were for 360s, and why would they pay over the odds for mine?

Then a thought occurs to me.

Maybe they bought the broken 360 and dvd drive with the intention of fixing it. Then when they realise they couldn't, they decide to buy an identical working one online (my one), then when they recieve it claim that it is broken (either RRoD or damaged in the post or whatever) and return the original broken one to me. I, being an unsuspecting idiot, would then apologise to them and refund their $300.
(or possibly after it arrives they claim it got lost in the post and so get me to claim on the insurance for the lost 360)

Of course it could be totally innocent and they may just want 2 x 360s, so I don't want to turn down the $300 'just in case'.

I figure I wait until the money has definitely cleared, then take pictures of the serial number on my 360 to keep as evidence in case they try and return the broken one.

The only difficulty is, what if they simply swap out the case and send me my original case, but with the broken 360 drives etc in it? How will I prove then whether it is mine or not?
Is there some other incontrovertible way I can confirm whether the 360 they try and return is mine?