I am full time pro and very hard on my equipment but because of necessity it is always in tip top shape when talking about function. I picked up a Leica M 10 in late July and it already has well over 30K on it. My cameras are used almost everyday. When I sold my Canon gear a couple of years ago it all went to other pros that I know and I think it is still all functioning well.
When talking used I know that I would rather buy a camera that has been used and well maintained over something that has been on a shelf.