I bought a used Wii from a Kijiji ad and have been super happy with it. One thing to look out for is people advertising their system as "comes with 8 games!!!" when what they really mean is that it includes Wii Sports (which is 1 "game" disc with 5 different sports) and 3 Virtual Console games of dubious quality. Keep in mind Virtual Console games are not nearly as expensive as packaged games, so including them is not a valid reason to hike the price of the Wii by very much.
In my case, I got lucky - not only was the Wii I bought virtually unused (looking through the play records revealed the person had only played it 5 times in a year), but there was a Virtual Console game and 4500 Wii Points left on the system that the person hadn't mentioned in the ad. I got a great deal and am super happy with the Wii.