$40-$70!?!?! You guys are going a bit cheapp... but I suppose you are talking about selling in store. I vote selling it on eBay, you can get £80+ (about $150?). And no, selling it isn't dumb. That's what I did with the Wii, I wasn't playing it, so I sold it.
150$ thats way too high. Right now I can buy a DS fat with an R4 2gb for 150.