About two years ago, I was on the hunt for a used vehicle, a truck to be specific. I wanted something that was still under warranty, without the new car tax, which means a "barely used" vehicle.
I rolled the dice on a used F-150 with about 22,000 miles on the ODO. Those miles were put on it a span of only 10 months. In the process of doing this, I saved roughly 13,000 off of it's then "new" price (yes, there may have been "buy new" incentives that I am not factoring in, but for the sake of my argument I am leaving it out).
5 - Rental companies typically take better care of their cars
The thing about a used car is, no matter what kind of mileage it has or the outside condition of the vehicle, you often do not know the maintenance schedule on these cars. I argue that off-rental vehicles (also off-lease vehicles) have a pretty diligent schedule as far as maintenance is concerned. A typical vehicle on a used lot may very well have a sketchy maintenance history, and you will pay more for that luxury due to the stigma of buying an ex-rental vehicle.
4 - These cars are usually in their manufacturer warranty period
Rental vehicles that are taken off the fleet and sold are typically between 20,000 and 30,000 miles. With this, you are basically getting the benefits of buying new with a 3 year/36,000 miles bumper-to-bumper warranty (albeit with very few miles left on the warranty) without much of the added cost. Many manufacturers will even let you pay to buy the extended warranty as long as you are still under the manufacturer's warranty.
3 - Rental vehicles are mostly highway miles
Anecdotal evidence here, but most rentals are driven long distance on the highway. Logic states that while it is indeed possible my truck was driven 22,000 miles in stop and go traffic in 10 months, it's probably not the case. As we all know, it's often not how many miles are on the vehicle, but how it was driven. A Fusion with 80,000 and a WRX STI with 40,000 miles are completely different beasts, probably flogged in completely different ways, with completely different repair bills. So while the rather young rental vehicle you plan on buying might have high miles, the kind of miles it has been driven may be insignificant.
2 - Most of the bugs have probably been worked out
All of my new cars, without fail, have had some issue that needed fixing within the first year. This is not uncommon, the modern car is a complicated piece of equipment, and things are bound to go wrong. The thing with a rental vehicle is that, when these problems crop up, these vehicles are usually fixed in a timely fashion so that they are able to be put back out to the rental fleet. A sitting vehicle for them is money lost. This is to your advantage as a consumer, as you will often get a vehicle that has had all the gremlin bits worked out that come with a new vehicle. If not, well see #4.
1 - You will save money
I can't stress this enough, there is a stigma against buying a used rental car. People do not like the idea of it and I can understand why. You are not sure if the majority of the people renting the car drove it like Sébastien Loeb on a rally course or your Aunt Loeb going to see the family in Middleville, Wherever, USA. My feeling here is that, it's a risk worth taking, due to the cost savings. I do firmly believe that it depends on the vehicle you are intent on purchasing. I would not personally purchase a sports car of any kind from a rental company. A sedan or family truckster? Sure.
Ultimately, buying new is a risk, as is buying any used vehicle. Take the proper care that you would with any vehicle you would consider used. Have a mechanic look at it for a quality Pre-Purchase Inspection. Hopefully you save a few bucks in the long run, and get a quality ride out of it.
A rental can be risky (What is the fastest car on the planet? A rental.) but rewarding if done with thoughtful research and good common sense.