Yea used GTA4 are a dime a dozen. The only Wii games I see alot used are MP3 (not much replay value, 1 or 2 playthoughs is all you need) and Red Steel (Sold over a million and is weaker game)
Zelda:TP is fairly common too.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut