because they are based on another person's copyrighted material.
Then everything is fair-use because everything is based on another person's copyrighted material.
if someone is selling the fics, that person is violating the copyright of the original author (but not necessarily that of the fic author).
Also not true. Copyright protects form, not content. If you publish content in another medium(for example oroginal was cartoon and you publish book), then no sane country's court will rule against you because another medium is good proof that work is not even just modified form.