If anyone has a chance of making it work, it's a company like Amazon.
I spent a bit of time last year trying to work on a a 3D model sharing site: http://www.fabfabbers.com/ - as a personal project rather than a viable business. In the end I found the idea of providing 3D development tools online much more interesting, and spent a bit of time converting OpenSCAD. I got a bit fed up with it in the end though, so it's just bit rotted since.
I don't know -- if it was one thing Amazon could do better than anyone else, it would be print-on-demand. But apparently they prefer to stick with traditional physical books.
Granted, unlike 3d printing a toy, or tool or what-not -- there are better (well potentially better) alternatives to print-on-demand: e-books.
I do find it kind of odd that they don't allow custom designs -- seems to defeat the purpose of 3d printing?
It seems fairly obvious that they don't allow custom designs. If you want to print your own stuff, go to a maker space. Quality control is important for a company like Amazon, and custom designs make that impossible. A small batch of items is the logical place to start a service like this. I'm sure over time they'll steadily incorporate proven designs onto the platform; probably they'll also let people submit designs and get a cut of the design, just like books. But, there will need to be a rigorous quality control process in place before the platform can be that open.
I spent a bit of time last year trying to work on a a 3D model sharing site: http://www.fabfabbers.com/ - as a personal project rather than a viable business. In the end I found the idea of providing 3D development tools online much more interesting, and spent a bit of time converting OpenSCAD. I got a bit fed up with it in the end though, so it's just bit rotted since.