I know this is a pretty old topic, but I just recently (in the last week) bought my first DS system.
I'm a big RPG/SRPG player, and to start my collection off, I've bought:
SMT Strange Journey, SMT Devil Survivor, TWEWY, 9 Hours, 9 Doors, 9 People, Theresia, Dragon Quest(s) 4, 5, 6, and 9, and I pre-ordered Devil Survivor 2.
My point is, that after researching, comparing, and buying a bunch of DS games...I only found one that was selling for considerably more than its release price...and that was Radiant Historia (and I'm talking new/sealed prices, here).

@ TC

I'm *far* from an expert or authority on making money off of video games, but, I've found that strategy guides are the better investment since they're usually cheaper than the games when they release - and - I was *amazed* at how many guides on Amazon (in new condition) were selling for considerably more than the games themselves.

A quick and recent example of this would be the hardcover Skyrim guides that I bought...I've already doubled my money on a few of them, and the rest...I'll just hold onto for awhile.

Also of note is that Doublejump is no more. Most of their guides were for Nisa (niche) games and I wouldn't be surprised if in a few more years they were selling pretty high prices.