The Collaborative International Dictionary of English v.0.48:

Sales tax \Sales" tax\ (s[=a]lz"t[a^]ks), n. [Sales + tax.]
   a tax imposed upon the retail sale of goods or the sale of
   services, usually collected by the seller at the time of
   purchase; -- it is typically calculated as a percentage of
   the price of the object sold, being commonly from 3% to 7% of
   the base price.
   [PJC]

WordNet (r) 3.0 (2006):

sales tax
    n 1: a tax based on the cost of the item purchased and collected
         directly from the buyer [syn: sales tax, nuisance tax]