I bought my PS3 off ebay and it's worked flawlessly since i've had it.
I even took a risk when buying it because it had been opened and repaired by the guy before.
It just depends on who you buy from and the luck of the draw, really. Sometimes you'll buy from douchebags and sometimes you'll buy from genuine people. This happens with many things, and used consoles are no different.
Unless you're buying a used Gameboy, SNES or N64 or something. If they can get electricity, those things will always work.