I find this whole topic amusing. I bet there's a ton of owners (maybe some of the "I'd Never Buy A Rental" guys) who've bought used cars which were previous rental vehicles, or used in a similar manner, and never knew it.
Besides the normal rentals from the big companies, dealers also rent car sometimes, then sell them as "used". If it's handled in-house there might not be any record of it being a 'rental'. Large companies have their 'fleet cars' for employees to drive which are essentially leased but more or less the same as rental cars ..and they wouldn't be tangged as a 'rental car'. Contractors and other companies buy pickups which end up on corner used car lots. They go to auction and dealers buy them. ..and so on. And what about off-lease cars? Were they treated any better?
When you go to dealer car auctions, the longest line/s of cars going through are rentals, fleet and off-lease cars. Usually there are 3 bays or more with an unbroken line of cars flowing through. They sit there on the bid floor for a grand total of about 10 seconds, the bid is done, ...and they're out of the bid bay and next car in its place. Dealers don't look them over other than a quick walk-around.
Those are some of the 'clean' cars and trucks at your town's used-car corner or dealer lot. People buy them all the time and never know one thing about their history. And generally speaking they work just fine.