We are working on a unique eCommerce site. This site is distinctive because when a purchase is made its not made by one person, but a group or "Collaborative" decisions. Individuals can add items to the shopping cart, but in the end the purchase is decided by the group in a “Collaborative” effort or Team Effort. So each team member is given tasks, inter-team messaging, can rate functionality with surveys, set milestones, rank specific features that are important to them. Another big part is that many aspects of the site allow for comments from members of the group. So many of the items in the application are “comment able” by the team. Is there anything we can buy (C#/ASP.NET/MVC) that gives us this functionality. Comment, Task, Survey, Rating, Messaging, Ranking Collaboration engine?
A lot of the features you mention are available in Microsoft's SharePoint 2010 platform - this software is intended exactly for the type of collaborative scenarios you describe including surveys, tagging, workflow, alerts, etc.
There are two versions of the product: SharePoint Foundation and SharePoint Server. The Foundation product is free and contains many of the basic collaboration and workflow features. The Server product contains much more - too much to list here - but is not free.
The 2010 product is built on ASP.Net 3.5. You can build both public-facing and intranet sites using SharePoint as the underlying platform. Also, SharePoint has many extensibility options, so if the off-the-shelf product does not contain a feature that maps to your particular requirements, you may be able to write custom code to extend it.
As a starting point, I would suggest you check out the features on the Microsoft SharePoint product page.