Carfax is good to get an idea of what's going on but that's it. If it doesn't go thru the channels they deal with or it isn't reported to them in a way they understand they don't show anything. There have also been cases where people have been upset because they indicated odometer tampering because one of the quick oil change places or other service either didn't give mileage or they were lazy and rounded mileage and it a number below the previous mileage listed or accidents listed for multiple body shop estimates or scuff repairs etc.

We've owned two former rentals that we know of and never had problems with either of them but yeah they are definately buyer beware. You don't want to end up with something that was driven dukes of hazzard style.

Usually when a new car dealer sells a used 1 year old car it's a former rental and if it's 2 years old it's a business lease return of some sort.