You don't need to improve your art. Selling is all about content. At the end of the day, what your art looks like in style means nothing to the buyer- It's the audience that matters. Like... if you have a computer geek market, you want vectorised art with lots of neons, and geeky jokes. The cute market likes chibis doing cute things, pastels and round objects. Doesn't matter HOW you draw, it, it's WHAT it is. You can be a beginner artist and still make it. It's all about the eye for design :)