i haven't looked at your listing to see whether or not the iphone turns on - but if it does, you can turn it on and go into the settings and view the info about the phone and the IMEI is displayed in there. you can photograph that screen or screenshot it and save a copy. obviously, the IMEI doesn't change regardless of SIM card etc. it's unique to the phone (which i'm sure you already know).

failing the things that tippy said - you can also photograph any identifying marks on a used phone. if there's scratches, no matter how small, try and take photos or two or three of them or any other identfying marks there may be - chances are, any other phone won't have the same marks in the same places.