It's true that their profit margins can be vanishingly thin, but that doesn't mean they don't make money. For some classes of items, they can sell at cost and

It's true that their profit margins can be vanishingly thin, but that doesn't mean they don't make money. For some classes of items, they can sell at cost and still make money, because their operations are allegedly so good that they can turn over the inventory before their own payment to the supplier is due. For example, say Amazon buys a book today and payment is due to the publisher in 30 days. They sell the book tomorrow at cost. Now they get to sit on the full price of the book for the rest of the month. In fact, take that money and buy another book, and sell it right away too. Keep that up, and you have a very big pool of money always sitting in your bank account. Money that can be profitably invested in other activities. Why would a publisher give them 30 days to pay? Because they're Amazon. It's good to be big.