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]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "marketplace":
   agora, amphitheater, arena, athletic field, auditorium, auto show,
   background, bazaar, bear garden, boat show, bowl, boxing ring,
   bull ring, campo, campus, canvas, circus, cockpit, coliseum,
   colosseum, commercial complex, course, emporium, exposition, fair,
   field, flea fair, flea market, floor, forum, ground, gym,
   gymnasium, hall, hippodrome, lists, locale, market, market cross,
   market overt, mart, mat, milieu, open forum, open market,
   palaestra, parade ground, piazza, pit, place, platform, plaza,
   precinct, prize ring, public square, purlieu, range, rialto, ring,
   scene, scene of action, scenery, setting, shopping center,
   shopping mall, shopping plaza, show, site, sphere, square,
   squared circle, stadium, stage, stage set, stage setting, staple,
   street market, terrain, theater, tilting ground, tiltyard,
   trade fair, walk, wrestling ring



Easton's 1897 Bible Dictionary:

Market-place
   any place of public resort, and hence a public place or broad
   street (Matt. 11:16; 20:3), as well as a forum or market-place
   proper, where goods were exposed for sale, and where public
   assemblies and trials were held (Acts 16:19; 17:17). This word
   occurs in the Old Testament only in Ezek. 27:13.

     In early times markets were held at the gates of cities, where
   commodities were exposed for sale (2 Kings 7:18). In large towns
   the sale of particular articles seems to have been confined to
   certain streets, as we may infer from such expressions as "the
   bakers' street" (Jer. 37:21), and from the circumstance that in
   the time of Josephus the valley between Mounts Zion and Moriah
   was called the Tyropoeon or the "valley of the cheesemakers."