I have entered the world of ATC... Many thanks to ~Tamara-Hawk for luring me in! ATC (= Artist Tradings Cards) are miniature artworks created on 2½ x 3½ inch / 64 x 89 mm cards (about the size of a business card). They are meant for trading.

I've also come across the name ACEO (Art Cards, Editions, and Originals) which seems to be a more known name for them. These are usually cards mainly created for the purpose of selling them, although some seem to use this term to mark their ATC as a unique (handdrawn) card.
You can find mine in this folder and the ones that are still available are in this folder.

I have found several groups here on dA where people post their cards; some trade and some sell, I'm only interested in those who trade.
Drop me a note or email if you're interested in one of them!
If you know of more great places to trade, please let me know.