This is an interesting question. It is likely very very high, 99% or higher for sure. Since the only limitations to what you can register are found on your keyboard, a ton of "crap" can make it onto the scene.
I guess just take a random sampling of 100 domains out there, does 1 out of 100 make it to see a sale? I would think the majority of domains registered are never intended on even being sold and never make it out to consumers or other domainers as well.