I have a client who is wanting to allow users sell their products. Basically, it would be very similar to how the marketplace here at Concrete5.org works. In short, there will be "vendors" on the site with their own marketplace.

So, there would be the ability for registered users to buy, sell and trad with other members. Orders and payment is forwarded to proper vendor.

Is it possible to build something like this?

Is someone interested in creating an addon like this?