I THINK the way it works is that you prove no one owns it in the first place. When I bought our moon acre we were sent a load of documents explaining what rights we did or did not have on our acre. Such as; if any one is to find any sort of minerals on our 'land' we do not have rights to those minerals. And that we cannot build on the surface of the moon - because it would damage the apearance.
The guy who owns the company that sold me my moon acre actually 'owns' the whole solar system as far as im aware. Abfandango - I was on ebay a few days ago and someone was selling an imaginary friend!, and I have found clouds for sale too.