Oh man, two different opinions, still confused and have to pay for this today. I can still back out, I can even add $350 and get a new Mark II if I want.
So as long as I have the paper from Canon with the serial number that is the same as on the bottom plate and in the camera software then there is no way he could have switched the body or something?
It just seems strange that Canon would put new body panels and grip and such, he doesn't say why they would do that, only that they did and this raises a red flag to me, as if he could be the one who changed the serial plate on the bottom or something.
I say add the 350 and get new one..then you know what you have.