This is more common than you think. It's completely legal for car owners to disable systems in a car. They're supposed to tell you that they've done so when they sell it, but nobody ever does, and there's no law that protects the buyer from such things.

People used to come into my cousin's shop and request him to remove dash bulbs from systems like ABS, SRS, etc... because Virginia State law requires that those lights cannot be on when performing the state safety inspection. Sometimes a $1200 car will come in with a $3000 ABS problem and the owner will choose not to fix the it. If the light is NOT on, the car will pass whether the system works or not because the law is not specific outside of that light. Same goes for Check Engine Lights, too. The owner of the car, or the mechanic may remove the light. The mechanic WILL MOST CERTAINLY require the car owner to sign a waiver, releasing them from liability for doing so. It's all perfectly legal.

Avoid this happening to you by first sitting in the car you're looking at. Don't let the owner start it for you. Put the key in the ignition and turn it to the ON position without starting it. All the idiot lights (except the signals) WILL light up for about 5 seconds if they work because every car has a bulb-diagnostic and self-check mode when the ECU sees power. If a light doesn't light up, beware. Could just be a burned out bulb, but if it's a major system that's not working, make sure the "bulb" is fixed before you show them a penny.