I'm hoping someone can help point me in the right direction for a project I'm working on.

I would like to make a website similar in functionality to ConstantContent dot com, or iWriter, or a similar freelance site.

Here are the main functions required:
Users can sign up either as a contractor or a client and create a profile.
The contractor uploads a digital product to the site, fills out a form with some details about their product, the product goes through an editorial review on the back end, it gets added to the database of available products for purchase, and the contractor automatically gets paid a commission when a client purchases their product.
Customers can preview the product before purchasing (with a graphic snapshot representing some of the text in the product, for instance).
Naturally, there's a mess of pages that would need to be included such as article topics for written content, and clients could select a category type and browse through the product previews for that category.
Clients can add special requests for items which are then broadcast to the community of contractors.

Hope I haven't forgotten anything huge; I've tried to be as clear as possible but I'm not a developer.

Is this possible to do with a CMS (like Drupal - probably not WordPress?) or is this a project that would require a lot of advanced coding? I'm comfortable with making basic tweaks to CMS code here and there if necessary, but not much beyond that.

If you read through all my blabbering, thank you so much and I hope you can help me. My budget is tight right now, so I really hope there's a DIY solution. If not... I'll just suck it up.