There's a big difference between individuals selling unauthorized fanarts of characters owned by well known companies and professionals, versus the other way around. Either way it's illegal, but one isn't nearly as bad as the other.
Also consider that in some cultures, professional creators are openly welcoming of small time artists selling fanart of their characters. Japan is one of them.
In Korea, for example, one convention had to hang up a huge sign requesting that professionals be discreet when purchasing fanarts of their own characters, lest the fan artists get intimidated. Creators there LOVE seeing fanarts of their stuff, even when it's for sale. It's still illegal over there... but no one considers it unethical or 'wrong.' I'm from that culture, so I dream of the day when my comic has become big enough for people to sell fanarts of it at artist alleys. If that ever happens, I'll buy the hell out of them and support the artists! (Unless they're making boatloads of money... in which case I'd work out a license with them and take my fair cut of the profit.)
Now, I realize not all creators feel the same. The creator's preference should be respected. If the creator/license holder is okay with it, then no one has the right to condemn the artists for selling fanart. If the creator is not okay with it, then no fanart sales of their characters should take place. The problem is that THAT can be difficult to find out. Ideally, there would be an easy way for fan artists to obtain permission/license for this stuff, but there isn't. (Apparently there is for music, though? Not necessarily easy, but at least feasible) Some companies are releasing official statements on their stance on fan art sales, though, so that's a start.