Jump the wagon, a new online auction site has opened recently and it's devoted 100% to video games. Why choose this one over ebay though you might ask? well there's no insertion fee for sellers! if that's not reason enough to try your luck on chasethechuckwagon.com then I don't know what is.
But either way, check out chasethechuckwagon.com as it has some pretty sweet NES stuff listed right now at very reasonable prices.