For example, drupal ubercart marketplace plugin.
In addition, i will need it to be scalable and highly customizable. For example, consumers will be able to group together in a yahoo groups/social networking fashion and then discuss and buy together as a group; merchants will have minimum volumes and time tools similar to those available on present group purchasing sites. Are there plugins for each of these things? Can they be integrated together? Is plugins to open source the only route to go? As there is a lot of customization going on here should i write from start?