Well, because of the large quantity of systems produced due to the success of the Wii, you can buy a system in good shape for little money, although of course it depends on the country you're in. In my area, game stores can sell you the console with composite cables and the power brick (but no controller) at 20€ or 25€, which is an easy deal for what these consoles can offer, specially if you get a GC compatible model.
But of course, I'm talking about a store and not a random person selling something over the internet... Personally I'd rather have a look myself before buying any second-hand console, because I've seen people trying to sell me some systems in horrible shape.
One example, a Nintendo DS Lite with a non-official shell (one of those cheap orange ones with "Naruto" printed on it). The guy sweared it was official, and didn't want me to check it out before buying it. Turns out that he didn't want me to see it because when you opened it, you could see the freaking screws poking out of the sides of the touch screen! How do you even manage to do that?!