Consider this a lesson in economics 101: College students who buy and sell their textbooks online can save upwards of $5,000 over the course of a four-year undergraduate education, according to an analysis conducted by comparison shopping and deal site Extrabux.com.

Jeff Nobbs, co-founder of Extrabux.com, compared the cost of purchasing college textbooks at campus bookstores with those bought through e-retailers. He also compared the sell-back prices from the same outlets and the rental prices for the same books.

He found that by buying used textbooks online at a site like Half.com, students will save almost 50% from the price of new books sold at the campus bookstore. When a student finishes using a book, he can most profitably sell it used to Amazon.com Inc., which offers credit on future purchases.

Nobbs, however, also recommends e-retailer AbeBooks.com, which, while it offers less money in the sell-back, pays the student cash instead of a store credit. Amazon acquired AbeBooks in 2008. He also compared the price of renting the same set of books for a semester at Chegg.com, a textbook rental service, and found that students who buy used and sell back books save 46% versus the cost of renting.

Timing, however, is everything, Nobbs cautions. The rule of supply and demand applies especially well when it comes to selling back college textbooks. Nobbs says the best times to sell used textbooks to a site such as AbeBooks.com is during July, August and January, when demand for used textbooks is high. The worst times to sell are April and December, which is when students traditionally finish their semesters and flood the market with used texts.

Holding onto last semester’s textbooks may mean lugging them home over summer break, but Nobbs says waiting until peak selling time will earn students as much as 20% more in the sell-back process.