What was a dream for offline retailers is actually common practice for online ones - to adjust price for each customer. How to do it? Cookies trace each computer's online navigation history - keeping a record of the sites you visited, the words you searched, where have you clicked, what's the computer you are clicking from, where is it located... If consumers don't want this to be known (apart from computer and location, that's unavoidable), they should actually clean their cookies once in a while (a very simple process) - but if they don't do it, then, this information is then used by online stores to actually taylor made their offering to consumers. Retailers may then personalize price, offerings, promotion, communication,... It is an incredible and powerful feature - a marketeer dreamland!
If you want to know some real examples, just follow http://blogs.smartmoney.com/advice/2012/06/26/how-online-retailers-personalize-prices/