I have to disagree with what you said Candyman.
If it is a nation chain rental agency, such as Enterprise or Budget, buying a used car from them is a pretty sound thing to do.
If something goes wrong with a rental car, it is fixed as soon as it does. I know this from experience. My father rented a car to come see me here in Minnesota. (He is from Alberta, Canada.) When he got here, the lever for the tilt colum wouldn't work, so he phoned an agency here and they told him to bring it in. When he did, they had it ticketed for repair, and gave him a new car for his trouble.
Beside having problems fixed as they arise, rental cars have all warranty and recall work performed, plus you get an excellent and extensive maintenance history. Just make sure you ask for it when you buy the car.
In my opinion, buying a used rental car is better than buying from a dealer or a private party.