In the United States, for example, the National Highway Traffic Safety Administration (NHTSA) has certain requirements and standards for the safety of used vehicles offered for sale. While the NHTSA does not specifically prohibit the sale of a car with a broken driver's seat belt, it does require used car dealerships to disclose any known safety defects to potential buyers.
Therefore, if a used car dealership in the United States is aware of a broken driver's seat belt, they must disclose this information to any prospective buyers before the sale. Failure to do so could potentially lead to legal consequences for the dealership, such as civil penalties or even criminal charges.
It's worth noting that even if the sale of a car with a broken driver's seat belt is not explicitly illegal in a particular jurisdiction, it could still be considered a safety hazard. For the safety of both the driver and passengers, it is advisable to get the seat belt repaired or replaced as soon as possible.