Only the County Department of Public Works (DPW) may post signs that have any legal authority to limit or prohibit parking along County streets and rights of way. DPW does not post official signs on private fences.
While there is no legal statute that prohibits a property owner from posting a sign on their property, they have no authority to remove a vehicle from public property or public right-of-way access. If a property owner actually had a vehicle towed from property that is not owned by them, the vehicle owner could sue for any costs or damage to their vehicle.
The public can safely ignore this unofficial sign posted on a private fence on West Point Ave in industrial Princeton at a popular beach access destination.
UPDATE 12/9/14: County Code Compliance required the removal of the "reserved parking/ tow away" signs on the Princeton fence near the beach on West Point Ave. because a Coastal Development Permit is required in order to impede public beach access in this way.