We have had quite a few requests to find a way to share a design with specific people without making the design public. In the last round of website updates this is now possible with a hidden model view state. It is relatively easy to do and means that you can sell the object to a customer whom with you share a private URL that includes a security code to access but still allows you to include (and conceal) your mark-up.
You are all now well aware of Noah Improbablecog Beasley's Open Source Jewelry project on Kickstarter, and you may know of Thingiverse, where Makerbot(ers) share there files to be extruded from hot gooey plastic, you may even know of Content Central, where Solidworks types (or just anyone) can upload/download 3D models of just about anything (mainly mechanical parts).
Shapeways has had the 3D Parts Database for a while, but now sharing your 3D files on Shapeways has just gotten a whole lot easier.