Amazon's interest in web-ad patent is not surprising.  After all, they have a goldmine for ads.  When a person buys a book, that person is making a significant investment of his/her time as well as money.  If I buy a book on stock options, you can be sure that I am interested in stocks and likely to respond to stock related ads.  So ad space on some of Amazon webpages are of high value.  Since ad space value can differ across pages, auction model works best.