They want me to pay for a car they didnt check

I'm very annoyed with this branch, when I got there to pick up the car, no one was available to check the car before I took it. So anyway I left with the car and it started giving me problems with the alarm from Thursday until Friday so I took it back. When I got there some guy checked it and found a dent which I definitely didn't cause, so obviously due to their carelessness no one checked when I left with the car and all of a sudden it has a dent that I must pay for. Someone just called me to say that I am liable because they don't have record of that dent. How do they know that the agents didn't put the dent on record because they are careless, they didn't even check the car before I left showing how negligent they are. I will NOT pay for something I didn't do and will be going to social networks and everyone I know to tell them never to use your services EVER!! BUNCH of *********** staff...