Until you find out that the cheapest source also waits two weeks to send your book. And sends it by a method that takes another four weeks. And doesn't package it properly so it arrives with the spine broken and water damage. And sends you a used book with pages missing. Or might just accidentally send you the wrong book but doesn't have a customer service contact.

The internet used to be a great place for bargains. Now "cheapest" is almost always the wrong choice.