Don't trade it in to Gamestop. You won't get half of what it is worth, especially for an older system like the DS. For example, the DSi XL (worth much more than the DS itself) only goes for $25 on Gamestop. Platinum goes for $22 and so does Black.
I think you could easily get more on Craigslist or eBay. If you're not looking to do much work, then I recommend maybe selling the two games to GS for $40 cash and then selling the DS elsewhere - that way you're getting more money and won't be saddled to GS when it comes time to buy.
- J Rokk likes this