Every store is best at something and worst at something else. Here is the honest breakdown, including the cases where the option everyone tells you to avoid is actually the right call.
AbeBooks — best for older and international editions
A marketplace of independent booksellers rather than a single warehouse. Depth is its advantage: obscure titles, out-of-print editions and international printings show up here when they exist nowhere else. Shipping times vary by seller, so order early.
ThriftBooks — best for common introductory titles
Enormous used inventory, consistent condition grading, and free shipping above a low threshold. If your book is a first-year survey text that a million students have already taken, this is usually a strong price.
eBay — best for the absolute lowest price, if you are patient
Individual sellers clearing their shelves price to move. Buy-it-now listings avoid the auction risk. Check seller feedback and read the condition description carefully, since "good" means different things to different people.
Amazon — best for new editions and speed
Rarely the cheapest for used copies, but hard to beat when you need the current edition quickly. Worth scrolling to the full list of marketplace sellers rather than accepting the buy-box price.
Chegg — best for expensive books you will never reopen
Rental economics only work above a certain price point. For a $250 text you will never reference again, renting is clearly right. For a $40 paperback, buying used and reselling usually beats it.
Better World Books and Biblio — best for humanities
Smaller catalogs, but frequently the low price on literature, history and philosophy titles that the big warehouses do not stock deeply.
The campus bookstore — genuinely best in three cases
It is the most expensive option almost always, but not always the wrong one. Buy there when the book comes bundled with a required access code for online homework, when the course uses a custom edition that genuinely differs, or when you need the book today because class started and no shipping will arrive in time. Paying more to not fail an assignment is a rational trade.