I want to say this in the nicest way possible because I love that you are working hard and practicing a lot trying to improve and kick out sculpts. I really want to see you grow and become an amazing sculptor.
I think that if you are going to try and sell your sculpts at this point it's better to do characters that no one has done before but there is still some interest in. I say this because everyone, including the top sculptors out there have done a Dark Knight Joker. So you have a lot to compete with and a lot of people have seen some just amazing sculpts. That sadly brings out the flaws in yours more.
So my advice would be to focus on other characters that interest you, try and fill a hole that no one is touching rather then jumping in a big pile with some of the top names in the business.
I hope that makes sense and is in someway helpful. I say it because I love your work ethic and I can't wait to keep seeing you grow as a sculptor. I don't want to see you get burned or turned away because you are having trouble selling your work when you have to compete with so many others and you are just starting.