Used copies are where the real savings live. These are the marketplaces worth searching, and what each is actually good at.
ThriftBooks
A large centralized warehouse operation with consistent grading and free shipping above a modest threshold. Strongest on high-volume introductory titles: general biology, psychology, intro business. Condition tends to match the description.
AbeBooks
A network of independent booksellers with unmatched depth on older editions, international printings and specialist academic titles. Shipping speed varies by seller. The place to look when a book is not a mass-market survey text.
eBay
Individual sellers pricing to clear space, which produces both the lowest prices on the internet and the widest variance in accuracy. Filter to buy-it-now, read the condition text rather than the headline grade, and check seller feedback.
Better World Books
Used copies with free shipping, funding literacy programs with proceeds. Catalog is smaller than the giants but frequently competitive on humanities and social science texts.
Biblio
Another independent-bookseller network, useful for the same reasons as AbeBooks and worth checking as a second pass when a title is scarce.
How to actually use them
Search all of them by ISBN rather than title, because the cheapest one genuinely differs book to book — there is no marketplace that reliably wins. Compare the delivered price including shipping, not the listing price. And order early enough that a slow independent seller is not a problem.
The other side: selling
When the term ends, buyback services bid competitively for the same book and offers can differ by a wide margin. Comparing buyback offers is the mirror image of comparing purchase prices, and it is worth the two minutes.