I "definately" think you could advertise for commissions. my only advice is if this is your"only" drawing you have that is this high quality, get a few more drawings under your belt so you can put together a portfolio and have multiple pieces to show off to potential clients. Do a few head shots and full body shots of different breeds and colors (I know these are black and white drawings, but you should still be able to tell a chestnut from a bay from a palomino). If you also want to offer horse and rider combos or tack included, provide some samples of those as well.
I think your overall composition is VERY good and I actually really like the shading a lot. I much prefer a high contrast drawing to a flat monotone drawing. It looks like you have outlined the horse with a solid dark line, which makes it look less 3 dimetional. I used to do that same thing (still do to an extent). Many artists will say that if there is a highlighted area right there at the edge, leave it light. Don't try to outline it. I actually personally like the outlined look, but if you truely are going for a totally realistic drawing, you may want to soften the outlined areas. But as I said, I personally happen to like that look and I think it makes your drawings uinique. I have no training at all short of some of the advice I have recieved in here and from friends, so I am not an expert on technique at all. I just know what looks good to"me"....lol.
Otherwise, practice makes perfect! I started out doing free sketches for people on myspace and then they started asking to buy the originals. I literally sold my art for only $15 for a long time before I gradually increased my prices. You mentioned the paper being of poor quality. You can justify raising your prices when you start using better quality materials, but for now, start low (like literally $15 or $20) and see how much interest you get. If it seems like too much, you can always raise your prices.
Good luck and keep drawing.