I don't trade games in, ever. That's a nice deal though.
I'm normally the same way. But with so many games being remade or now available digitally on newer consoles (Wind Waker, VC games) or just being flat out made obsolete by newer entries in the series (Mario Kart Wii, Wii online service dying tomorrow), I'm taking a serious look at getting ride of some of the games collecting dust for no reason on my limited shelf space. If I can get $15 for MKWii, I'd take the money and run. And I'm not at home so I can't check now, wasn't Monster Hunter Tri Ultimate on sale for $20 on the eShop not too long ago? If it still is and I can get $15-18 back from selling the disc, I'd do that and just go download it, netting myself more Club Nintendo points.