I suggest asking the creator of the fan art if you can sell stickers with their work on it. Even though it is fan art and they got inspiration for it from another source that has copyrighted their work, their work in itself is intellectual property.
When you ask, face the chance that they themselves may want some profit out of you making money from their work. Or, they may be ok with you selling it. However, the bottom line is if they don't want you to sell then don't sell. Doing so without their permission can lead to infringement on their property and at the very least, a very very angry artist.