Surprisingly, a lot of people tend to put the cart before the horse when it comes to selling things online. They decide they want to open an e-commerce business, daydream about how much money they can make, and think about what the site will look like. While they have a passion for the store itself, they see the products they would sell as secondary.
As with anything, to do it well, you have to believe in it. There are opportunities out there to make a profit by meeting an unfulfilled demand. Or maybe you can capitalize on opportunities to sell unique products that you make yourself, such as baked goods, gift baskets, or clothing. Know what you want to sell and what makes it special enough that people would want to buy it. The rest will come together more naturally if you use that as your starting point.