Is this simply because you can make sure it's maintained properly? Or something else?
I definitely value my decade old car, which I bout new, more than what I could sell it for.
Yes, because you can ensure that it will be maintained properly. Generally speaking, people don't treat their cars very well, and there is a certain level of risk that you assume when you purchase a used one. In effect, you are gambling a bit.
We'll use my recent purchase as an example. I bought a 2007 Toyota Camry with 93,000 miles on it for $12,000. I could have purchased a brand new Camry for $18,000. Up front I saved that $6000, but most of that $6000 goes up in smoke if the transmission eats itself. Now I'm in it for nearly the same as if I had purchased a new one, and I'm still not covered against further repairs like I would have been had I chosen the new one.
No. It is never cost-effective to buy a brand new car, that is an emotional (or otherwise influenced) decision, not a mathematical one.
http://www.mrmoneymustache.com/2011/11/28/new-cars-and-auto-financing-stupid-or-sensible/
Except in the real world, cars, even Toyota Corollas, break, and they do so in expensive ways and your risk of that happening increases with the age and milage of the car. If you're not factoring in that risk to your calculations, you're doing it wrong.
Not only that, but many cars need extra money thrown at them for maintenance items, particularly at around 100,000 miles. So add a couple thousand on top for that 90,000 car for potential brake work, tires, transmission servicing, timing belts, basic tune-up stuff like spark plugs, coolant drain and fill, etc. and you're THAT much closer to what it would have cost to purchase the brand new car that needing none of that, nor has the risks associated with neglectful and abusive prior owners.
I'm not saying that you're wrong, I'm saying it depends. To completely write off new car ownership as an option to consider without running the numbers based on your unique situation is folly.
I don't know if it's ever cost effective to buy a car brand new. no matter how long you are going to keep it. You can get a car 1 year old with less than 10K miles for a significant discount off brand new prices. And that is for a car that is mechanically good as new, and still under warranty.
Why didn't the original owner want to keep that car? Was there something wrong with it or did the car simply not meet the person's needs? These are questions you have to explore, but yeah you could save a ton of money that way, but you have to be on your toes to be sure you're not getting someone else's mess.