Best place to buy a used Nintendo DS?

As a little girl I played Pokemon on my Gamecube quite a bit. I'm now 22 and just realized, "hot damn, I miss Pokemon!" It's just too freaking cute not to play. However, I just bought a PS3, and don't know if I want to blow that much money on another gaming system right away. So I thought maybe I could buy a used Nintendo DS, especially as I'm not a huge fan of any other handheld games. Pokemon is about it. However, I'm wondering where the best place to buy one is, because buying used can be rather nerve-wracking. Tips/suggestions?