The Question of Fanfic arises on a regular basis. The following is a re-telling of an event affecting Marion (Zimmer)Bradley, which highlights the issue.
Marion was a champion of "fan fic" and used to openly give permission to her Darkover fans to write stories in it, as long as it was non-commerical.
Marion read one story that was to be published in the "official 'zine," and found it followed the same story line as an element in her next novel. She liked some of what the woman in question had done, so she sent her a note saying, "I like what you did. Would you mind if I incorporate it into my next book, as I'm already doing a similar thing?" She offered to give acknowledgement to the woman in the novel.
The woman wrote back demanding she get a co-author's credit and half the money for the next book and told Marion she would sue if she didn't get it.
Marion had to throw away a manuscript that was 45% finished. For those who don't know, Marion's health is very bad and she is now looking at every novel as if it may be her last. She was very hurt by this and hated to have to give up months of work because of this one fugheadded idiot. But her agent and lawyer insisted.
FAQ answers attributed to Raymond E. Feist are copyright by Raymond E. Feist.
It should also be born in mind that the answer given was only applicable on the date written, and to a specific question. You may find further, similar questions, in the FAQ.