Based on what you describe, i.e., a single rocker-type switch controlling both on and off modes by rocking the switch either up or down, I would say you have (or might have if you buy the house) a version of remote-controlled switching.
To verify, simply remove one of the switch plates. You should find that, instead of regular 120v wires, you have what looks like heavy gauge phone wire.
Essentially, what you have is 24v DC current running in those wires. Somewhere in the house is a relay. Sometimes all the relays are located in a central place, often next to the circuit panel. Other times they are next to the item they are turning on and off.
Pushing the switch upward causes the relay to go to the "closed" position, completing the circuit and turning on whatever it is that it is supposed to turn on. Pushing the switch downward causes the relay to go to the "open" position, turning off the item.
Next time you look over the home, take off the switch plate, then let us know what you find.