I spent ages yesterday trying to find the link for a report I read a while ago - it was on dangers of using car seats in different cars (it was talking about from mums car to dads to gran's etc and saying that people are wrongly assuming that because the seat was recommended for one car it would be safe in the others).
If you are using parent provided seats you have to take on trust that the seat has not been in an accident (if they got it second hand they won't know for sure). If its an older seat it may be one that's no longer on sale cos its not as safe as other designs.
You are also responsible for checking the seat fits safely in your car; even if the parent had it fitted in a store the same seat may not fit so well in your car, eg buckle crunch may occur. As the Which report says no car seat is universal yet; it explains that the seats and seat belts are different in different cars.
I have bought the car seats I use from kiddicare - they will check the fit in your car before you part with any money. Although worryingly, Which? tested car fitting services and found a number of places like halfords & toys r us did not fit car seats correctly.