The Collaborative International Dictionary of English v.0.48:

Jobber \Job"ber\, n.
   1. One who works by the job.
      [1913 Webster]

   2. A dealer in the public stocks or funds; a stockjobber.
      [Eng.]
      [1913 Webster]

   3. One who buys goods from importers, wholesalers, or
      manufacturers, and sells to retailers.
      [1913 Webster]

   4. One who turns official or public business to private
      advantage; hence, one who performs low or mercenary work
      in office, politics, or intrigue.
      [1913 Webster]

WordNet (r) 3.0 (2006):

jobber
    n 1: someone who buys large quantities of goods and resells to
         merchants rather than to the ultimate customers [syn:
         jobber, middleman, wholesaler]