I'm mostly an illustrator and have many pencil drawings in my portfolio. They have very different subjects: from fantasy dragons to common landscapes, but it's curious that often they (despite they variety) used to be found by the same keyword - sketch - and bought then. Just a peice of advice for fellow illustrators: if you have pencil drawings in your portfolio, don't forget to add the keyword "sketch" to all of them, this simple action will bring you many sales.