> No one is forcing you to buy a specific used car.
In a hypothetical situation with no mandated seatbelts it could take decades for the market of new cars be close to 100% with seatbelts at best. And of course much longer for the used cars market. So yes, many buyers in the meantime would essentially be forced to buy such a car, simply because at their price point and locality there isn't one available with a seatbelt.
> (nobody would buy separate aftermarket seatbelts for a used car)
I would assume most people who want seatbelts in the first place would buy a car that comes with seatbelts, even when buying a used car.