When Amazon launched, the standard contracts between publishers and bookstores had the stores paying 60 days after books were shipped to them. This made sense for conventional bookstores which would often have books on their shelves for months before selling them; but Amazon typically turned over their inventory every 30 days, meaning that -- even after the inevitable delay between charging a credit card and getting the money -- they received payment for books on average 2-3 weeks before they had to pay their suppliers. A lot of Amazon's early growth was funded by this negative working capital requirement.
Now that Amazon has expanded from books into CDs and DVDs and electronics and clothing and toys and food and computing services, of course, it's quite possible that their cash flow picture is very different.
I thought Amazon was the example everybody always used.