Education costs, especially textbook prices, are outpacing inflation and becoming a serious financial burden for college students. Because student textbook use is linked to academic performance, making textbooks more affordable and accessible is one important way to enhance learning outcomes. Online booksellers allow student-to-student textbook sales, which can lead to substantial savings for students. This study demonstrates that the prices of used science textbooks on Amazon.com demonstrate a clear supply-and-demand pattern, with prices increasing at the beginning of a semester when supply is low and falling at the end of the semester when supply is high. We show that students can save 20%–33% on science textbooks by timing online textbook purchases carefully. We estimate savings of $452 over the course of a 4-year college degree by purchasing textbooks early. To enable students to save on science textbook purchases, we suggest that instructors notify registered students of the required course materials during the class registration period (near the end of the semester preceding the class). Such savings can greatly improve the affordability and accessibility of textbooks for students, which may lead to enhanced learning.