Many people have very good experiences with rental car sales. We buy them for our salespeople who put a ton of miles on for a few years and then re-sell them.
In the world of used cars the most likely sources are off-lease and off-fleet. I find that off-lease may be less abused but have very poor maintenance.
I'll also add that the used car market (not necessarily including private sales) is very efficient. Prices move very logically and consistently and any glut being sold off by Hertz is going to be consistent with the market.
