But a market for governments does exist. People "buy" or support their governments through elections and immigration.
Edit: On second thought, perhaps you were talking about not a particular government, but a system of government. In that case, immigration is the only choice.
> People "buy" or support their governments through elections and immigration.
True, but elections are plagued by the public good problem, and immigration is in most cases an extremely large transaction cost. There is also obviously some pressure on governments from the threat of violent revolution, but that's an even larger transaction cost and less certain endeavor.
Edit: On second thought, perhaps you were talking about not a particular government, but a system of government. In that case, immigration is the only choice.