There is no single cheapest textbook store, and any article that tells you otherwise is guessing. Used inventory moves constantly, so the seller with the $18 copy of your biology book this week may have nothing next week. What stays true is the method: search by ISBN, check several marketplaces, and buy early.
The stores worth checking
AbeBooks aggregates thousands of independent used booksellers, which makes it consistently strong on older editions and international printings. ThriftBooks holds a huge warehouse inventory of used copies and is often the best price on common introductory titles. eBay swings wildly — sometimes the single lowest price on the internet, sometimes nothing at all — because it is individual students clearing shelves. Amazon tends to win on newer editions and anything where you want fast shipping, and its marketplace sellers are worth scrolling past the first listing to see. Chegg is a rental play rather than a purchase one. Better World Books and Biblio are smaller but frequently undercut everyone on humanities and older academic titles.
The single biggest saving: buy early
Used textbook prices are driven by supply, and supply collapses in the first week of classes when everyone buys at once. The same book routinely costs noticeably more in early September than in mid-August. If you can get your reading list from the syllabus or the course page before term starts, that alone is often the largest saving available to you — larger than the difference between any two stores.
Search by ISBN, not by title
Titles are ambiguous. Publishers reissue textbooks every two or three years, and a search for "Campbell Biology" will return a decade of editions at wildly different prices. The ISBN is a unique identifier for one specific printing: 10 or 13 digits, printed above the barcode on the back cover, on the copyright page, and usually on your syllabus. Searching by ISBN is how you guarantee you are comparing the same book across stores.
Consider the older edition
Prices for the previous edition typically fall off a cliff the moment a new one ships, often to a tenth of the current price, while the content changes very little. The risk is homework assigned by page or problem number. Ask your professor — many will tell you the older edition is fine, and a large number of students never ask.
Remember the second half of the transaction
Buying cheaply is only half of it. At the end of the term, buyback services compete for the same book, and offers vary by tens of dollars between them. Factoring resale in changes the math on whether to rent or buy in the first place.