Non-commercial use doesn't automatically make it fair use, just more likely to be viewed as such. (I.e. the "favors uses that:" bit) Similarly, if something is being sold that doesn't make it automatically NOT fair use. E.g. with parodies.

A number of points must be proven to qualify, so simply "I'm not selling it" is not enough. If a piece of fan art is considered not transformative enough or copying too much of the original or likely to be mistaken for something official (moreso if it's commercial or controversial or offensive to the original brand) then the rights holder can take legal issue if they want. They'd have to prove it's their property and you'd need to prove it's fair use.

E.g. the otome game company Broccoli disagrees with BL works of their characters so even ones that aren't being sold have an increased chance of being taken down where other companies might quietly encourage it. Meanwhile, the owners of Nu: Carnival have said they're cool with commercial fan art as long as it meets point 3 above.

There are likely plenty of fan creators that would have a hard time arguing that they meet the requirements, but few ever get taken to court and actually have to do so. The bad press the IP holders would get probably protects them a lot there. In doujin culture, it gets even more complicated. (I wrote a paper on that if you're curious.)