I can think of a few baseline things that would pretty much be necessary for the system to function at all, so they'd probably be fairly universal. For example, the integrity of the contracts would have to be upheld or else the contracts themselves would be worthless and the system would fall apart. The right to property falls into a similar category. You cannot trade what you do not own, therefore property rights must be an absolute in a trade-based system. Since you cannot enter a contract or own further property if you do not first own yourself, self-ownership would also be a necessary condition that cannot be removed. A considerable number of basic human rights could probably be further derived from the combination of property rights and the principle of self-ownership. For example, assault or murder would be a no-go because you are damaging that person's property, ie their body. So those three principles are likely to be essentially immovable pillars of Extropian law, non-negotiable by virtue of their necessity. When you further consider how those three principles interact, a surprising amount of common law could reasonably emerge.