How easy is it to create your own online bookstore?
Very, very easy. You just have to be an Amazon Associate to do it, and be willing to only sell Amazon's products.
Amazon's service is called aStore, which lets you create ... a store! It's still in beta, but it only took me about 5 minutes to build a simple shop with only nine products: Peat's Web App Books of Hotness, which probably gives you a good idea of what you'll find there.
Beyond my simplistic demonstration, you can add pretty much anything in the Amazon catalog (not just books), lots of sidebar goodies for people to oogle while browsing, and a reasonable amount of customization. Good stuff.
Amazon -- so hot right now.