Caveat emptor.
You gotta check it thoroughly. I am not sure what the card carrier is but I am guessing the slot needs repair. That's a really vague description of what the problem is. It's also the first time I've heard that on any Nikon body. Remember, these pro bodies can take a beating so you have to figure out what really the problem is. If it doesn't write on to the card then that could me problem with the electronics- it can be a simple problem but can also be something more complex.
I just got a used D700 body just recently so I had done research on the prices. D700 bodies in great condition 9/10 can still sell for more than a grand. I have seen a couple with their shutter replaced sell for more than 600.
It can be a steal, but it can definitely be a headache. My two cents worth.