Fanart (at least in the U.S.) is a curious beast. It kind of exists in a legal gray area that, in theory, could cause a great deal of trouble for everyone involved. That said, I have almost never heard of the potential ramifications ever actually coming to pass. Creators know that fanart helps grow and foster their fan community, and fanartists get an opportunity to expand and grow their craft, so in a sense, it is mutually beneficial. The few times creators have taken a stance against fanart, it hasn't been worth the backlash from the community.
If you're going to sell fanart at cons, here are some good general rules. Please note that I am NOT a lawyer, and this advice is all based on my experience as a con vendor and should not be taken as legal counsel.
- Read over the Artist Alley rules. Some cons have restrictions on how much fanart you can have at your table, usually around 50%. Some cons don't allow fanart at all. Even if the original creators won't shut you down, the con might if you violate their policies. They may also have additional rules about what types of fanart you may sell, so this is definitely something to check at every con you apply to.
- Avoid labeling the art. If, say, you draw Fullmetal Alchemist fanart, don't put the name "Fullmetal Alchemist" on the page, or even "Edward Elrich" or anything like that. Some cons will actually disallow that.
- Don't trace official art. You'd be surprised how many people try to pull that, and this IS something that can get you shut down. Tracing others' artwork and trying to sell it as your own piece crosses the line from fanart into actual plagiarism, and it is NOT okay.
- Write, "(c)[year of creation] [your signature]" somewhere on the image. Yes, it's fanart of another property, but YOU OWN THAT FANART. Even the original creators can't legally use your image without your permission (although some still try).
If you want more information on the subject, I highly recommend looking up The Game Theorists' videos on the subject. They cover the situation more comprehensively than I.
Rock on, guys, and never stop drawing.