Yes, a used car seat can be a safe option, but there are a few things you should check first.
Make sure you know the history of the seat, how old it is and whether it has ever been involved in a crash. You should also check that all parts are included, that the harness and buckles work properly and that there are no visible signs of damage.
It is also important to make sure the seat is still within its recommended lifespan and that it is suitable for both your child and your car.
If you do not know the history of the car seat, it is better to choose another one.