As a powerseller and top rated seller, eBay will be more likely to side with you, since you have a good reputation. If the OP is a paypal advantage member, you also get special support, which I've had a great experience with.
Before I sell any iphone on eBay, I will open it up and put marks inside of it, on the underside of the EMI shields. The buyer won't notice this if he opens it, and he likely wont recognize them. Another alternative is to put hologram warranty stickers on the board, which can't be transferred to the bad board.
Please keep in mind, the buyer may not be shipping to his own address if he intended to scam you. It's a common practice for scammers to have packages shipped to a neighbor that isn't home during the daytime.
A good way to tell if it's you're iphone or not is to look at the chrome ring on the headphone jack. The chrome ring on 3g iphones is brittle, and worn very easily. With excessive use, it will dull and chip, if your iphone wasn't heavily used, and his was, then the chrome ring will show it. Most people forget to transfer it when swapping the back housing for a new one.
Also, verify the S/N on the SIM slot with the S/N printed on the circuit board. The S/N on the circuit board is still on a removable EMI shield, but usually most people don't know about it. Check for fingerprints inside the phone on ALL metal surfaces.
It's the black sticker on the right top emi shield.
http://blogs.pcworld.com/staffblog/archives/ifixit-iphone-3g-teardown.jpg
Go to ifixit for disassembly information. You'll need any suction cup to disassemble the phone, I used the one from my sirius radio car dock a couple times.