I lost my micro card for my "New" 3DS and I wanted to sell it but no one would buy since there's no micro card, so does anyone have the default files for the "New" 3DS (I brought a fresh micro card and want to put default 3DS files inside of it). Also i've been looking but I haven't found anything.
A 3DS system will take a micro SD card and automatically format it. There are no default files on the SD card, it's only used to provide extra space for storing save files. You can pull an SD card out of a 3DS and put it into a computer to look at it, there's nothing special there. You can also swap out SD cards like you would change a memory card for only PlayStation or Nintendo consoles, using multiple SD cards to store games with a single 3DS console. I've done this myself before when I had too many save files and ran out of space.
You can test that the SD card works by using settings to move files from the internal hard drive to the SD card, or saving games to the SD card directly. If those files still work, you'll know the SD card is all set to go.