It's too early to look into marketability for you. Do a lot of life drawing and study anatomy, color theory, etc. and work your ass off for a few years before you worry about that. As it stands now, you have to be CRAZY good before you can think about making a decent living in the illustration world, so work on getting your technical skills up to par before you worry about selling your work.
Art, in general, is a very competitive field. With the internet nowadays, people don't need to hire a local artist to do a job. They can outsource it to anyone in the world. On a positive note, you can use this to your advantage. All you have to do is create a following for your art and you'd be set. As for art school, personally I would avoid it unless you have a full ride. There's nothing in art school that you can't teach yourself. I would invest in a Wacom tablet if you don't already have one. Start doing some digital work before you start college because that is part of the illustration field that you need to have a good grasp on. When you have that down, start creating t-shirt designs and selling them on sites like RedBubble or even enter them in a few daily t-shirt websites like TeeFury or RIPT Apparel. As for the traditional side of illustration, try doing some sketch cards. I really like what you did on this Captain America piece: [link] . That would work really well on a sketch card.
What exactly is a sketch card and how do I go about doing it? I'm sorry, I'm 18 and even though I've been doing this my whole life I'm still very new compared to a lot of others. Are they something I could sell on here?
Sketch cards are 2.5''x3.5'' cards (same size as baseball cards) that are miniature pieces of art. Artists create them to sell or trade online. eBay is a pretty big marketplace to sell them. You can get your sketch cards to be featured in packs of trading cards for companies like Topps, 5finity, Marvel, etc. You can e-mail these companies to get a shot at creating sketch cards for them. The pay is low (minimum of $1 per card) but they'll send you blank cards with their logo and info on them to work on. Sometimes you'll receive cards back (returns) or they'll give you blank sketch cards to create whatever you want on them to sell. Some artists make $80+ per card for doing commissions on those blank sketch cards.
Trying to sell your art online is very competitive and on this site everyone is expecting very cheap prices. As for what people would want you might want to look at the Job Offers forum and browse to see what people are asking for. On a site like this expect lots of commissions based on anime and video game characters.
I suggest you start making full pieces of artwork instead of assignments and sketches. People won't be able imagined how a finished product of yours will look if there isn't any.
As for me personally if I was interested I'd like you to do more black and white ink drawings on small cards and sell them physically but that's just me.