Due to overwhelming interest in the source-code I've decided to attach it here in the frist post.
Maybe some of you can collaborate, or do your own twist on it.
ASP.NET MVC2 (.NET 4 & C#)
Entity Framework as ORM
SQL Server 2008 (Express used for development)
Scripts for creating database and inserting some dummy-data is included.
Don't expect clean and tidy code. Some places it's a real mess...
UPDATE - 12.10.2010
The reason for updating this first post is unfortunately not the reasons I hoped it would.
I'm sorry to say I no longer have the time or energy to uphold the devotion and work required to finish the marketplace site. Being in a full time job, having a life and everything that includes simply does not allow for a project of this scale to be developed in a timely fashion. It's with regret I'm prematurely ending the project, and I'm sorry for disappointing you by doing so.
In addition to the lack of time, it's also a lack of motivation - reason being the introduction of WP7, already existing similar solutions for Android and Apple has it's closed-up AppStore.. I'm just not sure I see the potential I once did.
If anyone would like to have a look at the projectcode, I will more than happily zip it down along with the database script. Please PM me if you're interested.
Once again, I'm really sorry.