As far as selling fanart, is really depends on the characters you are depicting. I would never touch any North American copyrighted character, but there is a much different attitude regarding Japanese Animation and Manga. In Japan, it is quite common for starting artists to draw, and sell as doujinshi, characters that don't belong to them. So, if you chose to create and sell Anime Fanart, it is extremely unlikely the creator will come after you.
That being said, there are companies in North America who actually 'own' the rights to these characters. So while it is 'technically' copyright infringement, and they 'technically' can come after you. I have never seen, read, or heard of that being done. Companies will turn a blind eye to the point where you will see Fanart in Newtype and other magazines.
As with most other cons, you will have to sign a waiver taking responsibility for the artwork you wish to sell. Just in case lightning strikes, the con is not responsible.
As for other stuff you can't have in ArtShow/Artist Alley: you cannot have adult material in plain view, and you cannot sell images you did not create.
Hope this helps!