You cannot have transferrable property rights if contracts aren't enforceable against third parties.
I think you're begging the question. Maybe
you can't have transferable property rights without enforcing contracts against third parties, because you have a particular belief about the nature of ownership (i.e. that it only exists to the extent that observers are contractually forced to acknowledge it), but that doesn't restrict the rest of us who aren't burdened with that belief.
Okay, fair enough. But whatever that other mechanism is that permits transferrable property rights, unless it's carefully rigged not to, it will apply to other types of rights as well. In any event, Libertarians really are split on this issue. There is no one "Libertarian view" on this.