How Amazon.com undersells Best Buy, the Apple store, and almost everybody else

Unless you live in Kansas, Kentucky, New York, North Dakota, or Washington state, you'll pay no sales tax on many purchases from Amazon. This gives Amazon a huge—and largely hidden—price advantage over most other national retailers.

You'll get an especially good deal at Amazon if you're making big purchases and you live in an area with high taxes. In Chicago and Los Angeles, for instance, state and local taxes add up to 9.75 percent, the highest in the nation. Sales tax is 9.5 percent in San Francisco, 9 percent in New Orleans, and it's above 8 percent in Houston, Dallas, Las Vegas, Philadelphia, and Atlanta. The general rule: If you're choosing between Amazon and any other large national retailer, you'll almost always get a better deal at Amazon because of tax savings.

Why doesn't Amazon charge you sales tax? It has to do with the regulations states use to determine which companies must collect taxes. According to Quill Corp. v. North Dakota, a 1992 Supreme Court ruling, companies are only required to collect sales taxes from their customers when they have a presence in the state in which they reside. If you buy something from the Web site of a company that has physical stores nearby, you'll most likely have to pay taxes. When you shop at online-only stores, you pay tax only if the store has substantial operations in your state. Since Amazon's headquarters are in Seattle, you have to pay taxes if you live in Washington State, and because it has warehouses or other facilities in Kentucky, Kansas, and North Dakota, you've got to pay taxes there, too.