We rented a car seat from the car rental company once.
The car itself was nice but the car seat that came with it looked like it had definitely seen better days. Maybe it was even made in the 80s, judging from the oddly patterned fabric it was covered in.
It reminded me a bit of the Velveteen Rabbit. Like the Velveteen Rabbit, there were worn off patches on the arm rests from where little hands had grasped them. A few stains around the edges of the seat spoke of tasty little snacks previous occupants had enjoyed while strapped in to the car seat. Yes, it had been well used, but unlike the Velveteen Rabbit there was no love inspired in me by looking at it! Tossing it into a fire would have been an improvement.
From then on, I have preferred to buy an inexpensive booster seat to take on our trip which I don't mind leaving with the car at the end of our trip, if I want to lighten our load a bit as we travel back home. I like having a car seat that I know is clean and safe.
This picture at the top is one I took after we turned in our rental car at the Charles de Gaulle Airport in Paris just this last August. Those are the car seats they use, all piled up in heaps on the ground! I thought it was kind of yucky to think of strapping one of my kids into a seat that had been stored this way. I don't know that this is how all rental companies treat the car seats, but it's something to think about.