A plea to designers - You're not making things.

[CreativeCoPilot]

Meh. http://sendshapes.com/ - but even with that, you could just convert the G-Code signals back into a 3D model, if technical enough, and so inclined.

Look, you sell me a hard good, and I'll just convert it back to a digital model by scanning it. DRM has not worked, and it will continue to not work. I would love to sell digital models of stuff I make, but it's also kind of lazy. Just make the hard good yourself, and add a secondary operation to it that home-printers and hobbyists are incapable of recreating right now, such as decorative painting (even colored sandstone can't compete with a nice paint job).

Also, as internet access becomes faster and frameworks more complex, I'm sure we will begin to see more online dsign programs, such as Leopoly (http://leopoly.com/). Since you wouldn't have direct access to the files you're working on, the site's owner would then be in control of allocating rights management. Just something to think about.