I have just posted this on another thread and thought it may be a good idea to post it here too:
I hope this is allowed, please forgive if I am breaking any forum rules, but I think this is worth repeating.
ATC's are artist TRADING cards and that's the main issue that they are traded not sold. It's a new and emerging art form as the emphasis is on exchange not on monetary value.
Stirnemann's idea was that the only way to get an atc was to make and trade an atc.
That is the main and important difference, it's about one to one interaction, about community, taking money out of the picture, as such it is a unique concept a dig in the ribs to the money driven art world.
ACEO's are seen by many as "jumping on the bandwagon" by those just interested in making money and ignoring the heart of the idea.
Look here for a more indepth explanation.
http://www.artist-trading-cards.ch/trading.html
I would just like to add that many people who make atc's would not sell their work as, in their view, this dimminishes the art form and cheapens it, some artists make cards to sell (the card them becomes an ACEO)
hope this helps!
Colly