Sales tax

If an online retailer has a physical presence in a particular state, such as a store, business office, or warehouse, it must collect sales tax from customers in that state. If a business does not have a physical presence in a state, it is not required to collect sales tax for sales into that state. This rule is derived from a 1992 Supreme Court decision which held that mail-order merchants did not need to collect sales taxes for sales into states where they did not have a physical presence.

For a while, some big retailers with local stores sold their products tax-free over the Internet by creating separate legal subsidiaries to handle Internet business. However, lawsuits by several states and pressure from the Streamlined Sales Tax Project (a group created by states supporting the Streamlined Sales & Use Tax Agreement, discussed below) has ended that practice of avoiding sales taxes.

In 2002, state governments organized to fight back. Under the Streamlined Sales & Use Tax Agreement (SSUTA), 40 states and the District of Columbia banded together to simplify their sales tax codes in order to make sales tax collection easier. Under SSUTA, the collection of sales tax still remains voluntary. However, it is considered a necessary stepping stone to federal legislation.

Consider creating a list of credible international sellers whose shiping charges are less than the shipping plus sales tax charges of US suppliers.

Your question draws upon nuances in the law that are very old (about a state's ability to tax an out of state transaction), and relatively new (laws designed to promote and, more recently rein in, internet-based business activity. As a general rule, one state cannot compel a business in another state to collect sales taxes on transactions made by businesses in the first state to a resident of the latter state unless that business has some sort of presence in both states. This is why a small company in South Dakota, for example, with no presence in Washington will not be required to collect sales taxes for Washington even though the purchaser lives there. Where the business --such as a national retailer-- has a presence in Washington, sales taxes made through an Internet storefront located in another state will be taxed.
Out of state Internet-based purchases that escape local state sales taxes may be a thing of the past very soon. While the Internet Tax Freedom Act passed by Congress in 1998 sought to promote tax-free web commerce, more than 44 states have since signed on to the Streamlined Sales & Use Tax Agreement, which will require uniform collection of sales taxes among member states.
Keep in mind that most states still require residents to pay so-called "use" taxes where sales taxes were not collected. Have these laws been extensively enforced? To date, not widely, at least on individual small and non-business purchasers.

Simply, they are out of reach of the instate tax authorities. Some states, such as Massachusetts take use taxes very seriously as many taxpayers cross to New Hampshire to make capital purchases (think TV sets), although they do not stop cars at the border, only ask on the sate tax return if you have made purchases subject to the use tax. It is probable that most ask the same questions regarding us taxes, so in effect you would be committing tax fraud by not reporting your internet purchases. Congress should clarify internet based transactions so all taxpayers know where they stand.