Can I sell characters that I created in character creator 3?

so if I create a character with character creator 3, and create the clothing in marvelous designer,
can i sell it in the marketplace?

Unless it is pertaining to a Video Game etc or a Direct Sale for such then technically no, you would have to go through the proper channels to get permission from Reallusion since they use proprietary topology on the characters created with it. There is a certain percentage they earn and it is however okay to sell in their marketplace and content store but outside of that you would need their permission to do so.

See the following article: Can I sell the characters I create with Character Creator 4? (reallusion.com)