After having been a member of a Print On Demand (POD) site for a few years, the topic of fan art came up frequently. I don't do it myself, but over the years have collected a number of good articles explaining why posting fan art on a site where it's for sale often results in removal for infringement. Most of these articles are very recent. If I find any more in the future I may add them here.
Lawyer written article on fan art: http://www.gurwinskeyboard.com/fan-art-tribute-or-tribulation/
Video Game Fan Art Initiative: http://www.ign.com/articles/2016/01/20/redbubble-launches-video-game-fan-art-initiative
Star Trek guidelines for Fan Films: http://www.startrek.com/fan-films
Fan art article re: copyright and trademark. https://sandpaperdaisy.com/2015/11/24/should-you-respect-copyright/
Fan Art and Copyright: http://graphicleftovers.com/blog/fan-art-vs-copyright-infringement-legal/
Fan Art article by attorney, specifically about conventions: http://www.sethpolanskylaw.com/2016/06/10/intellectual-property-the-wrath-of-cons/