This is not true. When a seller sends in their inventory for us to store for them, they are required to attach their own assigned UPC to the product. We can track what product it is and what seller it belongs to via that special UPC/asin. Also when you purchase from a specific seller, you are prompted to leave them feedback, it would be unfair to leave negative feedback for a seller who didn't sell you that particular item.
since when is a UPC seller specific? I just quoted the Amazon.com site, so I'm not entirely sure how they handle these things.