I think they all do, mostly through the channels that own the rental companies. For instance Hertz is connected with Ford, so selected Ford dealerships take would take rental fleet cars, recondition them and put them on the market.
And IIRC there is a similar connection between Budget and GM. I know a guy who bought an off rental Toyota that was in the Budget fleet through a GM dealership in Ottawa...not sure which one.
If I were you, I'd check with the rental companies directly. They should be able to tell you the process.