macuser: I think this isn't a courier issue...
OP thinks the buyer switched the new iPhone with a broken one and is now trying to return it for a refund.
He's trying to get photo evidence of the serial number without alerting the buyer that he doesnt trust him
I know this sounds elaborate, but perhaps if you asked the buyer to take a video that includes both the screen being broken and a macro shot of the serial number on back.
Don't use the eBay message tool for it though, tell him the standard procedure is before he sends it back for refund, he needs to send you this video (via Dropbox or another sharing tool) in 1080P and the damage and the serial need to be clearly visable - tell him the same video needs to include any courier package damage as the point of it is to make sure there is proof for courier.
once he has sent the video verifying the serial, you will need the phone to be sent back to NZ for verification.
When it arrives, if it ever does ...check there is no signs of tampering (perhaps they switched the LCD) so there potentially could be wear marks on screws (new exposed metal, rounded center from it being stripped by the wrong size screwdriver) or pry marks around the edge of the display where it would require a tool to lift the LCD.
But the only way that it could be damaged is if it was courier damaged. Unless the buyer is outright blaming the seller for sending a broken device intentionally. In which case it is one persons word against the other. A video showing the items being boxed may have helped with this, but who does this.
The other thing is that you don't know if the buyer has opened it up and switched the internals. Alternatively they may have dropped it the first day they got it.