A large number of undergraduate in the U.S. sell their textbooks back to campus bookstores every semester. These used college textbooks in turn constitute a large proportion of the textbooks sold in subsequent semesters. These used book sales have profound effects on the behavior of textbook publishers and students. Publishers are powerfully motivated to price textbooks higher because they cannot recoup their development costs by selling a new book to every student who will be assigned to read the textbook. Publishers are also powerfully motivated to bring out new editions in order to "kill off" the used book market that depends on the earlier edition. Students are encouraged to pay higher prices for textbooks because they know that some textbooks can have significant resale value. Other students do not buy the textbooks at all because of the high prices.