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. SNIP. 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 alot of customization going on here should i write from start?

What you have tried to say is perhaps using a Forum like activity for your ecommerce marketplace like, groupbuyingsite.com or foodzie.com or etsy.com. In my knowledge, there is no such exact plug-in. With proper coding, the custom forum can be built and can be integrated with the site. If you want further suggestions, you can send your requirement here also.

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