It's easiest if you can leverage knowledge you already have. I also tried stores selling items I knew very little about, and it was much harder. =)

There are many tools for finding what's selling though. The best ones are paid, like Terapeak (http://www.terapeak.com/), but you can also search eBay completed listings and get a sense for what's selling.

Finding the right product(s) is definitely the hardest part, though -- you're absolutely correct. Not only is it hard to find products, you also have to find products with margins that enable you to make money. For some of my sales, I only make about $1-2 dollars. Which is something I need to improve, for my own business.