Some nonfiction books have very limited appeal in the used marketplace and should be avoided: Time-Life series, Readers Digest condensed books, and most National Geographic titles. These are wonderful books, but not in demand after they’re sold new. Sports, art and coffee-table tourist books are all dicey for the online seller. When new, many of these books sell strongly, but hardly anyone wants to buy a used one for themselves.

Business books are fertile ground for online sellers, as long as the material isn’t too dated and the book was not a mega-best seller. Sometimes events can make a business book obsolete overnight. For example, after the stock market meltdown of 2000, books on stock day-trading went in the tank.

Self-improvement is a strong category, as long as it’s not a fad book by a celebrity.

Biographies are a marginal category most of the time. Memoirs of political figures, especially dead ones, are best left in peace. The history category produces several blockbusters every year, but used copies sell slowly a few years later. Certain sectors of history are the perennial winners: The Civil War, World War II, Vietnam, and anything on the Confederacy. Don’t bother with World War I.

Certain nonfiction categories, like fiction, must be very current to have value. Don’t buy travel books unless it’s this year’s edition. Same with law books.

Cookbooks is a huge area and many sellers even carve out a specialty within cooking. Specialized titles such as bread baking, diet cooking, natural foods and ethnic are areas to explore. One type of used cookbook to avoid is microwave cookbooks, which are nearly impossible to get rid of.

Used textbooks are overlooked by many online sellers. True, textbooks quickly drop in value when new editions are published every few years and values plunge. But if you have a source of very cheap textbooks, bulk purchases can be a good way of picking up some valuable stock. Usually a few out of ten will be worth decent money.

And vintage textbooks, particularly graduate-level mathematics and economics, are sought after by academics. Just about everything is collected by somebody. University professors, authors, and researchers purchase vintage texts.

You can usually ignore several categories of textbooks without missing anything valuable: introductory sociology and political science texts more than a few years old, study guides, and introductory economics and computer books. After a few years, these books are basically worthless.

Certain types of textbooks, however, hold their value much longer. Biology texts are revised less frequently than any other text category, so the value is steadier over time.

See a real-time list of the most highly sought after used and collectible books.

Read more free articles on selling used books profitably online: http://www.weberbooks.com/selling/selling.htm

Steve Weber is author of "The Home-Based Bookstore: Start Your Own Business Selling Used Books on Amazon, eBay or Your Own Web Site" (ISBN 0977240606). Got a question for Steve? Send to: steve_weber@yahoo.com