A lot of cars are fixed with dummy airbags, or no airbags at all. If there is no airbag, the light should come on, unless a resistor was installed to trick the system. Even then, the light should come on as part of the startup unless the bulb was pulled.
I would run, not walk away from this one, unless you can be 100 percent sure that there is an airbag, and the bulb wasn't pulled. Don't take the guy's word for it, get proof.