issues with the battery car doesn’t want to start

Bought a car from them 4 months later the car started giving me problems with a battery. I can’t leave the hazards on if the car is switched off because the car won’t start, you can’t even switch on the lights if the car is not on. once u leave the lights on if the car is off the car won’t start. I called the sales person and tell him about the issue he told me theirs nothing he can do. 5 months later the battery is worse. Went to check the life span of the battery only to find out that the life span is 10% it’s been on 5 months since I had the car but the battery is on 10%😳. Called the sales person and he told me that the car is a second hand so I must buy my own battery,Was so disappointed.