The Collaborative International Dictionary of English v.0.48:

Scalper \Scalper\ (sk[a^]lp"[~e]r), n.
   1. One who, or that which, scalps.
      [1913 Webster]

   2. (Surg.) Same as Scalping iron, under Scalping.
      [1913 Webster]

   3. A broker who, dealing on his own account, tries to get a
      small and quick profit from slight fluctuations of the
      market. [Cant]
      [1913 Webster]

   4. A person who buys and sells the unused parts of railroad
      tickets. [Cant]
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

scalper
    n 1: someone who buys something and resells it at a price far
         above the initial cost; "he got theater tickets through a
         scalper"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "scalper":
   adventurer, agent, big operator, clerk, floorwalker, gunslinger,
   lame duck, margin purchaser, operator, plunger, sales agent,
   sales engineer, sales force, sales manager, sales personnel,
   salesclerk, salesgirl, saleslady, salesman, salespeople,
   salesperson, saleswoman, seller, selling agent, shop assistant,
   shop clerk, smart operator, speculator, stag, store clerk,
   ticket scalper