As a former photog for a large Midwestern newspaper, I can tell you that Pro's view thier cameras as tools to be used and not as objects of veneration. They are used until they malfunction and then repaired, if they malfunction often, they are traded for different gear that works.
I would never purchase gear that a Pro had used heavily, now matter how well it may have maintained.
That said I have purchased two M bodies and lenses from Pro's. One was a portrait photographer that also collected cameras and to fund his switch to digital Canon, he sold me an M6 and two ASPH lenses, all still in boxes with recipets and Registrations. The other purchase was from a Long Island Times photographer that had purchased an M7 and never used it, but with digital, had no use for it, the camera was new and a good value.
But a camera or lens used every day by a Pro is subjected to much more usage than any amature would ever experience. There is a reason that Pro cameras and lenses are designed to a higher standard than Prosumer or amature equipment and it shows in the price tag.
On the other hand a well heeled amature photographer is a great source for used equipment. In many cases the stuff is barely used and essentially new, the owner not being able to bear the thought of damaging such expensive equipment. -Dick
I have 3 Nikon F that were 4 to 7 years in Vietnam. They are doing fine. All they require is maintenance.