Start a barter site with over eight thousand extensions to functionality, plus uncounted site designs to choose from. In case you're not familiar, joomla is an open source content management system for community driven websites.
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The Barter Sites extension gives your members the ability to post their goods or services in the barter directory on your site, send each other offers and messages, negotiate, and 'pay' with either direct trade or with your own private currency called trade credits. You can even issue lines of credit to boost trade!