I think your art is very good, but on the second and third images I would recommend making the lines stronger. The second one I would also suggest to draw the lines all the same shade, because some lines appear darker than others. The third one is very well drawn but the lines are being covered be the coloring, (I'm not sure if you did this purposefully or not, but in my opinion it would look better with the lines in front.) If you were to pick up on these miner details I'm sure you could sell them for more them for over 50k :3
Good luck.