I'm pretty sure nintendo dont allow people to develop and sell stuff for their kit without a license.
Actually it's a bit more blurry than that... You can develop for the DS, you can sell software for it as well, it's just the hardware(cartridge) that you will miss. But some homebrewn software have been sold online for the gameboy and gameboy advance in the past, it was not "games" it was a more devious usage of the gameboy, like music trackers (LSDJ- Nanoloop), digital oscilloscope or early embedded computing for robots.
What I like about developping for DS/Gameboys is the platform, quite limited somehow but consistent and standardized... and portable!