There are already a few companies dabbling in this space http://www.shapeways.com/ being the one I'm most familiar with. The most interesting approach would be if you could find a market niche that had idle capacity and a need for creative custom solutions that was currently going unmet.
The real chicken and egg problem with this idea is that you won't have enough designers and qualified production bureaus to form enough of a critical mass of purchasers until you have a critical mass of purchasers to make the service attractive to designers and production shops. It would probably work best if you start off with a small regional network of production houses that employed their own designers.
Big manufacturers already provide construction blocks, in standardized formats, for importing into design tools. From ICF forms to wallpapers. The 'marketplace' part is missing though, that's true. I don't even think it's a matter of technology, it's just that the industry isn't ready for it IMO. Construction is a very conservative industry.