Nothing. Something like 80% of used cars re-sold at 6 - 18 months old are ex-rental. If not, where else were they going to come from?
Nowadays, the slightest damage to a hire car, bodily or mechanically, comes off your credit card. And engines are built to such tight tolerances that they are actually better run in by a variety of different drivers than by just one, especially than by an overly 'careful' driver. The sweetest, most reliable car I have ever experienced was an ex rental Mondeo, bought at 10 months old with some minor body damage to the roof in January 2009. That car served me, my wife, and then my ex-wife very well and very cheaply for nine years. Nothing significant went wrong, and even components like brake discs lasted a very long time. I don't blame salesmen for obfuscating the history of perfectly good cars when they know full well that stupid prejudice against ex-rentals would lose them a sale.