I tend to doubt that Hertz is listing cars for sale that are still available for rental. That seems like an odd thing to do, considering the car could sustain damage, not be returned, etc etc.

To me, "will be sold" more likely means that Hertz decided to sell the car after it's returned, and thus it's not allowed to be rented out any further. Once it's returned, they send it to the sales group who cleans it, takes pictures, notes the mileage, puts it out on a sales lot, and lists it on the site.

Just to give one issue with them selling a car while it was still in circulation - the cars on the Hertz sales site list specific mileages. What if you put like 10,000 miles on the car during your rental?