This is a gray area, to be honest.
I do not mind if the fan artist sells the stuff they make of other's work as long as they do not profit from it. That means the price they put must only cover the cost of production (usually just printing cost or shipping cost).
I believe the community has drawn a line that they shouldn't cross so they'll know what they're doing.
The key point to remember is this: Fan fiction and fan art are, usually, an infringement of the right of the copyright holder to prepare and license derivative works based on the original. This is almost without exception.
However, many copyright holders, for good reasons, tolerate fan art and even encourage it, but this should not be taken as carte blanche to do what you want with the source material. There are many lines that a fan artist can cross and wind up in legal trouble.