I have not, but I know of another Wash owner that did and it worked out well for him. The machine was well maintained and he had the opportunity to see it in operation before it was removed and he bought it. He did have a professional installation and got a lot of life out of the machine.
I would imagine being able to see the machine in operation and having the maintenance history is huge. Certainly not exactly the same, but has some similarities to buying a pre owned car - your experience could go either way.