Here's a personal character of mine:
My niche is definitely monsters and aliens. I could try advertising on Twitter again. There's just a lot of competition.
I was hoping to see an adopt you actually did finish and put up for sale...
I can only say the Axel character is very clear and clean. His contours and patterns are sharp and easy to tell. And he's all around realized. He looks like something I could see people buying. While simple, the abstract gray background with it's rough gradation also adds a tone to the design.
Keep that same energy if you decide to finish that rabbit. This won't guarantee a sale, but it's important to show you care. The appeal of a fully rendered base to less artistically inclined people is providing something they can use as reference for commissions. As much visual information as reasonably possible.
One of these designs I made to put up for sale. The other two are mine.
You might be able to pick it out but I think it's clear all of them are very close in time I spent. Try to match the quality of your own personal designs with ones you wish to sell. That's the best direct advise I can give you.
And yes, take it from me, selling them is quite hard. At pretty much all levels. I'm sitting at what many people consider a lot of followers and only
one person wanted to purchase the one design of the three. And if you do not have a significant following, it almost feels futile. Especially on FA or any other gallery sites where there's no means of signal boosting or reliable methods of self advertising (I believe reposting counts as spam on FA). It takes time and great consistency to be able to sell anything on such a saturated medium. So don't feel bad. We're all struggling.
This site may aid you a *little* bit.
https://ych.commishes.com/