Awh i love 6! I have some tips even though im not the greatest artist XD
1. Use really eye apealing colour palettes, especially greys and dark blues, people seem to like that.
2. Look at whats popular and see what people seem to be buying right around now, like if Emo is being bought, do some original Emo adopts!
3. Try to do line edits and make their lines really smooth, don't do anything too detailed if you don't know what your doing, although, it looks like you got that covered! I really like the clean edits on 6.
4. And something else to think about, do ufas/pick your price adopts, get a view on how people think your art is worth. Although, sometimes its good to put a price and stick to it.
5. Final advice, try to post often, but not too often that people get sick and tired of the same designs (you seem to run out of ideas fast) so try to do original ideas that you haven't seen others do.
I hope this helped and im excited to see some of your work in the future!
*follows*