I'm looking for an open source ecommerce solution that supports multiple vendor selling, in a similar fashion to Amazon Marketplace. I'm not sure this solution exists, but I'd be interested in some feedback.
I've looked at Open Cart, Magento and Prestashop, but none of these appear to offer these features.
You are wanting an Enterprise-level ecommerce solution for free. You aren't going to find one. I see you included Magento as a tag. As you probably already know, Magento Enterprise Edition starts at over $11k per year. That would be probably one of the best solutions out there for what you are looking for, but at a hefty price.
I can guarantee that Amazon has invested millions into their website. You just can't expect to find anything like that for free.
You can try Drupal + Ubercart + Ubercart Marketplace. I use it for a "place de marché" shop, with multi-vendor, and Paypal Mass Payment. There is a product based on osCommerce named oscMall who is free and open-source.
Hope I could help!