From what I see, almost all popular internet artists make cash on selling fanart, one of the highest earning of them - Sakimi Chan is doing almost only gross softcore porn fanart and is getting away with it.
This prominent artist youtuber claims that selling fanart is not only legal but also not wanting to sell fanart is uprofessional:
https://www.youtube.com/watch?v=RF7SlofTr4E
How is it possible? Did copyright/trademark laws change or something?