I asked a friend who knows copyright about a similar subject when I painted my dalek picture and his answer is that all fan art is a technical breach of copyright but that the owners generally turn a blind eye because fan art only makes their franchise more popular and suing would be public relations suicide, not to mention never ending.
This is not always the case and some copyright owners are very touchy about how their characters are portrayed.
I think however that the definition of fan art stops short of prints made for the express purpose of selling and you are at the point at which your people should do lunch with their people.
I realize that. I know that scouts for Disney frequently attend bazaars and confenscate and works that have mickey mouse/winnie the pooh/etc on them.
However, I personally see no problem with it if the picture in question is an original character that the artist (or someone else) has created. For example, I have done commissions of other people's original MMORPG characters... and of course, those are based off of WoW. Star Wars, etc. Since they are something that the person who commissioned it created, I saw no problems with it. But I would never accept a commission of a copyrighted character, such as Luke Skywalker or the like.
Fan-based character commissions havedbeen making me think lately. Yes, the character does belong to its creator, but that character would never have been created if the comissioner hadn't see the show/book in the first place. It's the "idea" thing that's getting at me. It's hard for me to explain.