If you live in the same state as a major retailer, the addition of sales tax can make a big difference in the total price of a build. With Amazon now charging sales tax in California (and starting in several other states soon), finding the cheapest prices is no longer a trivial task.
To solve this problem, PCPartPicker now provides fair price comparisons by allowing you to factor in sales tax rates for price calculations. On your preferences page, you can enter the sales tax rates for in-state retailers. Once those values are saved, all part pages, part lists, category lists, and markup will reflect the updated prices. When PCPartPicker chooses the cheapest price for a given part, it will automatically factor sales tax into account depending on the retailers and rates you have entered. (Note: sales tax rates are stored with your user account, so you will need to be logged in to have it included.)