quoted information: -
"Sales tax you pay in the U.S. is not comparable to Value Added Tax (VAT) levied by many European Countries. Sales taxes are levied by individual states, and the regulations regarding tax refunds differ from state to state. Contact the Department of Revenue (Sometimes called the Taxation and Finance Department) in the state your are planning to visit for more information. In many cases, you are required to have a State Sales Tax Exemption Certificate before making any purchases.
For more information please see the U.S. Bureau of Customs & Border Protection web site"