Selling a car based on misleading...

I bought a car in Autroprix a month ago. While choosing the car the sale rep was very friendly, and just to close the sale he offered everything. We were three people dealing with the sele rep. and the three of us understood they were going to put new tires on the car.
Not only did they not place the new tires on the car,but since day one I felt something wrong with one of the them. I took the car back to the dealer to have the tires checked and they said everything was OK. Two days ago I got a flat tire and whent to a repairshop where I was told that the tires were done.
I call Autoprix to explain the situation, but they just gave me excuses: they said that is not in their policy to add new tires to used cars (That is not what the sales rep. said when he was selling me the car), and that they only check the safety of the vehicule. Are tires in good condition not necessary for the safe operation of a motor vehicle?
I lost my confidence on Autoprix and its sales rep. So, if you don't know much about cars and you can't tell if they are selling you a lemon... just don't go there, and if you do make sure you get everything in writting.