Annie Lowrey reveals online retailer tricks:

Retailers read the cookies kept on your browser or glean information from your past purchase history when you are logged into a site. That gives them a sense of what you search for and buy, how much you paid for it, and whether you might be willing and able to spend more. They alter their prices or offers accordingly. Consumersin the few cases they recognize it is going on, by shopping in two browsers simultaneously, for instancetend to go apoplectic. But the practice is perfectly legal, and increasingly commonpervasive, even, for some products.

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