Yes, a dealer would sell you airbags, belts and all of the other stuff. Its better to go new. There are no laws that I am aware of governing the use of used undeployed airbags, but I have seen a car (01xc70) than had a used bag put in it only to be wrecked by the guy rebuilding it, it partially deployed (barely broke the surface of the steering wheel) If you sell this car with used bags in it or without a bag in it at all it might come back to haunt you in the form of a lawsuit if that car wrecks and the bag doesn't deploy because its used or not there. I was told by an FST (Field Service Technician) ((the guy who trains the volvo techs and are dispatched to sort out a queer electron in a problem volvo)) that each airbag has a s/n attached to it that corresponds to a vin number that the bag came out of. How they can tell this, I don't know. Go new its safer for you and those after you. Also, be pretty sure that the warranty on this car will be punched as VOID in the Volvo system. No Warranty means it could be very expensive down the road.

Volvo uses a decelerometer in the center console to trigger
the air bags. I'd have this all done by a dealer who is familiar
with all the details and has all of the manuals and can test all
of the components (the seat belts also have tensioner devices that
may need replacement and the side air bag system should also be
considered). You may also need special equipment to test the system
and reset it so you don't have a dash light constantly on. The seat
belts should also be replaced after an accident(they stretch).

I'm actually curious if a dealer would sell you the air bags.
Have you looked into that?