@Jeruba – most big brand rental agencies (Hertz, Avis, etc.) have recently changed policies where they are keeping their fleet up to and past 50K miles (this has been communicated to me by numerous managers at over a dozen rental agencies—so after hearing it several times I’m inclined to believe it).
So, a rental with only 30K miles on it was likely to have been a rental used at the dealership. This could mean it’s the car they let their sales manager drive (or the manager’s spouse)—as that’s a frequent “perk” of working for a dealership. Or, it could be the loaner car they used for their Service Dept.
As for me, personally, I’d never buy a retired fleet (Avis, Hertz, etc.) rental car because although they are maintained for safe operation they aren’t maintained the way my husband and I would maintain our cars (we take very good care of our cars, use synthetic oil, etc.etc.).. and they are driven HARD ..and as someone who travels a great deal for work I’ve actually experienced quite a few rental cars in very poor shape and even had a few break down on me. (Which, frankly, is surprising in a car under 25K miles..but yet, I’ve had it happen.)
I would however consider a loaner/dealer rental return. In fact, we bought one (with only 7K miles on it) for a substantial discount a few years back..I think it was a Toyota Corolla and it was wonderful. Perfect condition, even had the “new car smell” still…but it saved us thousands of dollars (and still came with the factory warranty).