The Collaborative International Dictionary of English v.0.48:

Bargainer \Bar"gain*er\, n.
   One who makes a bargain; -- sometimes in the sense of
   bargainor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bargainer
    n 1: someone who purchases and maintains an inventory of goods
         to be sold [syn: trader, bargainer, dealer, monger]
    2: negotiator of the terms of a transaction; "he is a hard
       bargainer"