Ok, so I am a writer, not an artist, so my opinion on this is slightly different.
So if your saying they took your comics, made a fan fiction of them, and are selling them as their own, that would so not be okay. Fan fictions should always be free to read. If they want to make someone pay for them they need to change the character names and alter the backstories somehow.
If your saying they took your comic and are drawing the characters in a single piece of artwork and selling it, that is borderline bad, just because they did not come up with how that character looks in their own head. As a writer, one person's drawing of my character will look different than another person's drawing. So they have some creative license in drawing any of my characters.
Honestly if their work is good enough I might hire them lol. (That would be assuming my novel took off and I now have money, which i don't lol)