Ask if the owner knows the camera's history. If the owner has had it since new, what did he do with it? If he was a pro, wedding photographer, etc. the shutter may have a lot of miles on it. If an amateur, the camera could well be a steal even if it is the early Pro model. That said, I bought a Pro from someone who used it for wedding for a while and as an amateur for most of its life, and it has been a very good camera for me.
juan