Basically, when you sign up for stumbleupon.com, they ask you about your interests. Then you click on "stumble" (a button that will be in your bar, up there), and you will "stumble upon" a bunch of stuff you're may be interested in. If you like it, you give it a thumbs up... if people have given your shop thumbs up, it will rank higher.
Sometimes I "thumbs up" my own items and then I get TONS of people looking at them. Good way to advertise for free!
Good job!