Ask the buyer of the other GH3 for the serial on his box (and camera). Then you know it for sure ^^. Because: As far as I know the internal serial dosn´t match the external one (at least it wasn´t the case with any GH2 or GH3 I had til now).

Hmm. You can also try to contact the Panasonic support (for an internal to external serial check - If such a thing is possible). You need to get into the service manual - Though: I don´t know if the internal serial is helping at all (maybe it´s just the motherboard serial or something else - Just checked my GH2 - There´s kind of a serial mentioned). Btw.: Use that method to get into the service manual:

http://www.dpreview.com/forums/post/51019354

On the other hand: If you get the info according the other GH3 AND the shop is accepting the GH3 (with the checked serial from the other GH3), your problem is solved anyway (no need to be unsure about it ^^). Btw.: Don´t forget to ask them if it´s enough to have the serial only on the box. Otherwise it´s a useless endeavor (and you´re wasting your time ^^).

that's an easy one.

On mac just open a .RW2 file with preview, then info (command + i) and in the last exif value (after the lens serial, case elect contacts) you'll have your body serial i.e. mine starts with X04+10digits.

For getting serious info though and with almost any model on the planet, the only real cowgirl way is ExifTool by Phil Harvey

Once instaled, on terminal - windows shouldn't be too different - type
exiftool -v file path

in the cascade of info search for (command + f)
InternalSerialNumber

there you go =)

That was grand, and nice to know whatever but I think the one I'm after is different, those WE ones. So I get it what Tschekoff was saying, the internal doesn't match external. Hmmm. I better write one on the outside in felt tip.