Amazon has been accused of engaging in 'dynamic pricing'. People have actually looked at an item at a particular price, went shopping some more, returned later only to find the price was now higher. Many companies engage in this. Crucial is a well-documented culprit. Dell also does it. Profile information is stored about your buying habits. These may be in the form of cookies, but many on-line retailers share information among themselves. If you look like an easy chicken to pluck, they will up the price if given the opportunity.