Some good points so far.
Check out Opendesk. https://www.opendesk.cc/
Colin which was featured on Core77 recently: http://www.core77.com/posts/74406/New-O ... ufacturers
My understanding right now for your idea is that a customer looks at designs on your website, finds something he likes, somehow gets quotes from local fabricators and ultimately makes the purchase at which point presumably a design documentation package gets sent to the fabricator which in terms makes it.
I don't know what's your level of experience in the manufacturing field but things don't usually work this smoothly when getting something made, even if you provide stellar documentation. There's usually quite a bit of back and forth between the fabricator and the designer/engineer. Material availability, tooling availability, equipment requirements, heck just the fabricator wanting to make sure they understand correctly slow things down.
IMHO, these kinds of on demand fabrication businesses work when say Shapeways or Protolabs have built up great software that are able to give feedback on manufacturability and are using machines that have a lot of freedom in what they can do. (You often end up in direct contact with someone at protolabs anyhow...). Or the flip side like custommade FH13 mentionned where a maker has a lot of freedom in how she builds.
One interesting business proposition would be an on demand manufacturer. IE you have a clearly defined set of manufacturing capabilities. Designers can work and verify their work with your business. Then, you have could have several of these small but uniform manufacturing centers throughout the world that can fulfill orders. Unfortunately, I kind of doubt the fact that you'd be closer would offset the potential savings from mass manufacturing. And by mass, I mean anything more than 5 of something at a time.