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Google built its $38 billion business selling ads based on how people search and browse the Web. Facebook, too, uses what it knows about its one billion users to sell targeted ads. But when it comes to what many advertisers value most—what people actually buy, or what they may want to buy soon—there may be no better data than the information in Amazon’s 152 million customer accounts. Since last year, the world’s largest online retailer has been packaging information on what it knows about consumers so that some marketers can use it to make split-second decisions about where to buy ads online and how much to pay for them. This automated process occurs on real-time ad exchanges that sell ad impressions as a person loads a Web page.