The Collaborative International Dictionary of English v.0.48:

Bargainor \Bar`gain*or"\, n. (Law)
   One who makes a bargain, or contracts with another; esp., one
   who sells, or contracts to sell, property to another.
   --Blackstone.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BARGAINOR. A person who makes a a bargain, and who becomes bound to perform
it.