Unfortunately, you are not allowed to sell fanart because it's a violation of numerous copyright and trademark laws. Basically, it's illegal to sell or not ask permission for its usage. It makes sense because it would deprive the original mangaka of their much-needed Royalties. However, in the USA and other countries, there is the concept of the Fair Use Doctrine wherein artists can use copyrighted material to show off their skills and for exposure but not for profit.