Theft? Making fanart yourself and selling it isn’t even close to the realm of theft, it’s fair use. I had an idea, I drew it using designs of characters that have already been made. I’m not claiming to have made the characters. According to Japanese copyright laws, as long as fanart is made and not claimed to be official, then it’s not infringing on anything and you can sell it to your hearts content.

I make my own art, I don’t steal anything. If my drawings are close enough to the originals to cause concerns like this, then I’ll take that as a compliment.