Shopping used cars is not the same as new. With new cars, all the advice given here is very valid because you're getting the exact same car no matter who you buy from, and it's a car that hasn't been used and potentially abused. With used cars, you really have no way of knowing what's going on with it. If you buy CPO, you're getting a car that has passed a lengthy inspection and carries a warranty. Sometimes that warranty is quite good, even, like another 5 years and perhaps 50,000 miles or so. Typically it's at least 2 years and 24,000 miles powertrain, often with a 1 year bumper to bumper.
Carmax does a major inspection and reconditioning on any car they sell. I've taken the tour of one of their reconditioning centers even and it's pretty impressive. I've also purchased a car through Carmax and it was immaculate. They come with something like a 7-day refund policy and 30 days bumper to bumper warranty, and have relatively affordable extended warranties available. Similar to buying CPO, you are paying for all that inspection and warranty too. A CPO or Carmax car will always cost you more because it's been reconditioned and carries that warranty.
The odds are generally in your favor to buy a high quality cheaper car private party or through a no-name dealer, but you then assume all risk upon yourself. It comes down to how comfortable you are with risk. The Prius, especially if you buy '07 or later (they revised the battery logic around then) or buy an '04 or later that's already had the traction battery replacement done is generally a safe bet on two fronts. First, they're very reliable cars to start with. Second, they're very boring cars to drive and thus generally only owned by adult drivers who don't beat on their cars.
So the odds are in your favor, but it's up to you to decide how comfortable you assuming the risk for yourself. In the case of the Prius, I know that in my area I can generally find a 2nd gen Prius with 150-250k miles either with a replacement traction battery or an 07+ model for $6-7k in what appears to be relatively good condition. By good, I mean I'm willing to overlook some paint damage, a few serious but non-punctured dents, and heavy wear in the trunk and/or back seats. Meanwhile a CPO Prius with 90k miles or a Carmax Prius with 120k miles could be $10-12k. For me, I'd rather spend less and roll the dice again, but then again I only drive 4000 miles a year and do not currently use my car to commute to work, so I am not at risk of having a broken down car prevent me from getting to work on time.