The idea that you only own something if every third party agrees to abide by the contract under which you bought it is appropriate enough for a bitcoin forum, I guess, but I have to say this is the first I've ever heard of someone trying to apply it in the real world. Did you come up with it yourself, or is there a school of thought out there that believes libertarianism actually does require enforcing contracts on people who weren't parties to them?
You cannot have transferrable property rights if contracts aren't enforceable against third parties. Libertarians are split about more complex cases. Ask this hypothetical to a dozen Libertarians, you will get a wide spectrum of answers:
"Fred and Jeff own competing sales businesses. Fred has a contract with all of his employees that if they leave him to work for a competitor, they may not contact existing customers for 180 days. This contract is valid and enforceable against those employees. Fully aware of this, Jeff hires away Fred's employees and directs them to contact their existing customers and try to get them to switch to his company. When the employees express fear that they might be sued or that they shouldn't break their word, he tells them not to worry about it. Fred hears about this from his customers. Does he have a suit against Jeff?"