I've bought quite a bit of used and NOS TMCC and Legacy on Ebay and the trainz.com auction site. So far I've had no major problems - virtually everything was accurately represented in the sellers descriptions. Some of the best deals are the items actually described as non-working. I usually buy stuff fully prepared to repair boards at the component level. So far, other than one bad EL backlight driver in a TMCC Aquarium car, everything else has turned out to be either bad wiring & connection or mechanical. Of course, that's excluding things like traction tires, bulbs and smoke unit wicking which I expect to do in anything used.
Jim