Both sell enormous numbers of used textbooks, but they are structurally different businesses, and that structure predicts which one wins for your particular book.
How they differ
Amazon is a retailer plus a marketplace with a single checkout, fast fulfillment and a uniform returns policy. AbeBooks is a network of thousands of independent booksellers — effectively a search engine across small bookshops worldwide, now owned by Amazon.
Where AbeBooks usually wins
Older editions, out-of-print titles, international printings, and anything niche. Because its sellers are real bookshops with idiosyncratic inventory, AbeBooks surfaces copies that simply do not exist in a warehouse system. Prices on superseded editions are often startlingly low.
Where Amazon usually wins
Current editions, speed, and convenience. If you need this term's edition delivered in two days, Amazon is generally the answer. Its rental and ebook options also cover cases AbeBooks does not.
Shipping and condition
AbeBooks shipping varies by seller and can take one to three weeks, especially from international sellers — a real risk if classes have started. Condition descriptions are written by individual booksellers and tend to be conservative and detailed. Amazon's timelines are predictable, and its condition grading is standardized but sometimes optimistic.
Returns
Amazon's policy is uniform and straightforward. AbeBooks returns run through the individual seller under a platform-wide guarantee, which works but is slower.
The practical answer
Check both, every time. For a current-edition introductory text you need next week, expect Amazon. For a previous edition, a graduate-level text, or anything unusual, expect AbeBooks — and expect the price difference to be large enough to be worth the extra shipping days.