I'm just copy and pasting this reply I made to the umpteenth person who has asked 'Can this artist really sell prints of their fanart?'
So, I'm putting it here so I don't have to write my opinion out again.
Correct me if I'm wrong, because I don't know. I'm just stating what I think and what I see.
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Well, many people on dA will submit 'fanart' that is either a direct trace or simply someone else's work manipulated, and that would cause problems because it's got nothing of their own in there. dA probably can't monitor every print that comes in, so probably just avoids all that legal mumble jumble to be safe. So, no. You can't sell fanart prints of dA, but that doesn't mean it's illegal.
But fanart is such a grey are because it is someone's art based on another subject. In Japan, it's fairly common for people to sell their work. They often do it in entire books, called doujinshi. Usually companies won't act because it's an effective form of free advertising and establishes something that is hard to fake - a fanbase. If you have fans spending time and money creating fanbooks (doujinshi) and fans who spend money on these fanbooks, it's often certain that this fanbase will spend huge amounts of money on the companies products because they are dedicated to that subject.
In the West, it's a little different. It's not so common, and we're more likely to ask 'Is it legal?' We're so obsessed with copy right, we're likely to doubt that a person can sell fanart. That's why Japanese artists are usually so cautious about selling their work to the West.
However, unless the fanartist is making huge amounts of money (which they won't be, I can promise you it's only really pennies and not worth the paper taking it to court would cost), or causing a great deal of slander to the subject, a company is unlikely to waste the time and money taking them to court.
It's such a grey area, with the art merely being an interpretation of something else, and really, artists have been doing that for years. Look at Francis Bacon's many, many studies and paintings of his version of Velazquez '

ortrait of Pope Innocent X'. In a way, you could call that 'fanart'. Bacon could never have seen Pope Innocent X himself, he's just basing it off of Velazquez work. But you couldn't honestly sue Bacon, it is Bacon's own interpretation of it - and isn't that what art essentially is? An artists own interpretation of what they see in the world around them?
So, yes. Most of the time a fanartist can sell their work.