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

OSCommerce is one of the best open source ecommerce systems. You should investigate it to see if it has any plugins that meet your other requirements.

Sorry, the title should have read, "What are my open source options?"

Hi Chris. Thanks for your reply. Are there any other options apart from oscommerce?

There probably are but you'll have to go searching yourself. OSCommerce is probably the most extensive with the most plugins.