The two best sources for used books are yard sales and library book sales. You'll quickly get a feel for which books will sell and which should be left behind. While you are cruising for books, by all means be careful to look for First Editions which are defined by the American Booksellers Association as books printed from the first setting of type. These editions are often more valuable than later printings.

You will also likely make more money selling larger trade paperbacks than the smaller and more cheaply printed mass markets. As you accumulate lots of books, pay attention to the way you store them. Be careful to keep them clean and dry, and to stack them carefully on shelving. Books may be stored in boxes but too much weight will damage the spines.

McBride Publishing puts out a wonderful set of pocket guides that would be worth purchasing.

I also recommend Michael Mould's, Online Bookselling: A Practical Guide with Detailed Explanations and Insightful Tips.

The best place to start selling books is on Amazon.com They have the highest sales of any online book seller and make it quite easy to sell through their site. They'll take a percentage of course, but you'll make money. You can start out with just a few books as I did, and when your inventory grows large enough, you can become a premium seller. You will pay them a monthly fee but will get additional services in return.

I have a list of additional links that should prove useful to the beginning bookseller.

Good Luck!