Update: This seems to play multiple systems (GameBoy/Advanced/DS/3DS)
This weekend my cousin told me he bought a 3DS off craigslist that was "hacked", and all he had was the SD card inside the system. No need to buy any emulator carts or anything like that. The guy that sold him the system did this all for him before selling it. So I looked into this and found it to be relatively easy to do...
What I used:
(you need an account to see links)
This site will give you step-by-step instructions on how to do this all. It's basically a bunch of files being downloaded and placed into folders. If you follow the instructions carefully, it's painlessly easy.
An SD-to-USB, something like this (though any adapter that lets you put files from your computer to the card will work)
If anyone has any questions, feel free to post them!