As long as the rider wears a helmet and follows standard safety precautions for riding a motorized scooter, it is safe to ride a used Honda Ruckus. The fact that the bike is used has no impact on its safety.
Motorized scooters have many of the same safety risks as motorcycles since they do not provide much protection in the event of an accident. The upside for the Honda Ruckus and other motorized scooters is that they do not reach the high speeds that motorcycles and cars reach with a top speed of 35 mph on a flat surface. There are always risks involved with riding motor vehicles, but the Honda Ruckus is not especially unsafe.