In many ways, if you're trying to sell an exact part, G-Code is already what you want for this. While there's lots of tools out there that can take GCode and render it, I don't know any that put it into a practically editable state. In that way, its like giving someone the printed schematic a the part. Sure, they can go tediously reverse engineer it and make a CAD drawing of it, but presumably, they're paying you because its faster for you to do that.
Of course, you can't prevent them from making as many copies as they want.
As a more practical matter, I'm not sure how useful this is. I use my X-Carve as an outlet for creativity. Boxing me into someone's exact part (as others said, on the exact same hardware setup) is the exact opposite of what I want. I might pay for someone's design to use a baseline for something I'm building, but I can hardly imagine just being okay with something out of the box. In that sense, there is a sizeable market for 3D models, or plans for woodworking items, etc. It doesn't seem crazy to do the same for CNC, but 1.) I'd do it outside of Inventables, as they're just one of many options, and 2.) You need to sell the plans (Vectric .crvs or SVGs for Easel or something), not the G-Code.
FWIW, when I find time to make it, I'm going to purchase these plans: http://www.derekhugger.com/colibri.html .