You could have the skills to draw a perfect replica of the mona lisa in ballpoint pen and I wouldn't buy because they're photographed. Either get a scanner, or guarantee being able to send the original artwork to whoever buys. Also never!! have lined paper for your examples. D;
Artwise, however, plain sketches in pencil or pen doesn't really do much. You need a little more pizazz. Color would be nice, even if it's limited palette, pops of color, or full color and even shading. Unique lineart could be a draw as well, as is, you have some scratchy habits (not chicken scratch, but could use some cleanup. Also all but one of your examples are in-profile, and the one that isn't is head-on, which are both terrible comfort zones you should push yourself out of.
I'll commend you on the different body shapes! You can tell they're all different creatures. :> I really like the front half of this guy, it actually has the most 3D form, but you lose it a bit in the back half. You have some good texture hints on this guy too. Keep pushing yourself!