The Collaborative International Dictionary of English v.0.48:

marketplace \marketplace\ n.
   1. an area in a town where a public market is set up; a
      market place; a market[2].

   Syn: mart.
        [WordNet 1.5]

   2. The commercial activity whereby good and services are
      exchanged; as, without competition there would be no
      market.

   Syn: market.
        [WordNet 1.5]

   3. The mechanism by which one finds a person to whom to sell
      or from whom to buy goods; the opportunity to buy and
      sell; a market[3]; as, to put one's goods on the market.
      [PJC]

WordNet (r) 3.0 (2006):

marketplace
    n 1: the world of commercial activity where goods and services
         are bought and sold; "without competition there would be no
         market"; "they were driven from the marketplace" [syn:
         market, marketplace, market place]
    2: an area in a town where a public mercantile establishment is
       set up [syn: marketplace, market place, mart, market]