Technically, selling fan art is illegal. You can draw fanart, share your fanart, but unless you have the permission from the original creator, you could be sued for making money out of it. So… you could be reported for copyright infringement. It’s tricky because you own the drawing itself, but not the character that’s on it.
Now, 99% get away with it for those reasons:
- People are unaware that selling fanart is illegal, so they don’t report it.
- A lot of creators don’t really care or see it as free advertisement.
- Sometimes the franchise is so popular and there is so many fanart sold all over the place that they can’t sue them all.
Some sites that let you upload art for sale have an agreement with a certain number of copyright holders to pay a royalty for fan art on behalf of the artist whenever a sale is made. I don’t know if it’s the case of Fiverr, but I don’t think so. Maybe a user that have more experience with Fiverr could help you more.