> However people who don't want to wear seatbelts generally only endanger themselves.
If they sell the vehicle, the decision was already made for the new owner (nobody would buy separate aftermarket seatbelts for a used car). So no, they also endanger other people. Mandating them outright is the right decision.
> 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.
If they sell the vehicle, the decision was already made for the new owner (nobody would buy separate aftermarket seatbelts for a used car). So no, they also endanger other people. Mandating them outright is the right decision.