Use Tax in US is levied on the use, storage or consumption of goods within the concerned state, for products purchased from outside the said same state. Use Tax is levied at the same rate as sales tax that would have been applicable had the concerned purchaser bought those goods within his state of residence.
By nature Use Tax is supplementary to sales tax. Buyers who did not have to pay sales tax due to purchase outside the state of sales tax are subject to Use Tax instead for use of the product within the taxing area.
In case sales tax has already been paid on the concerned goods at the point of purchase, usually there will be no further burden of sales tax on the same, regardless of where the use is taking place.
Purchases that attract Use Tax include those made in course of traveling outside the state of residence and brought or sent home, on line marketing through the Internet, mail order transactions, purchases on telephone etc.
Read: http://www.ftb.ca.gov/current/usetax.shtml
http://www.window.state.tx.us/taxinfo/sales/
http://www.boe.ca.gov/sutax/pam71.htm
http://tn.gov/revenue/tntaxes/salesanduse.htm
http://www.ucop.edu/ucophome/policies/sutm/
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