In a new car, you know exactly what you are getting, or are supposed to get, and if you don't there are laws in place to protect you. In a used car, you don't always know. Not everyone thinks to get a CarFax report, and even if you do, those are not foolproof.
For example, I have a friend who was looking for a new car early in 2009. Hers had broken down and she was in a crunch to find a new one. She came across a 2008 Ford Focus with low miles, and what looked like to be pristine condition. The price was great, so she jumped on it. A few months later, she began having some engine trouble, and when she took it in to a local shop, she found out the ugly history of her purchase. It had been a rental car for a few months before it was totaled. It was auctioned off and rebuilt, which is why it looked to be in such perfect condition. Would she have paid almost $18,000 for this vehicle had she known this? I mean, come on, would you?
Plus, there are other little things. When my husband and I bought our 2007 Nissan Murano in November of 2007, it had been a demo car. We thought it was the best of both worlds because it had been maintained by the dealership, was the new model year, but was thousands less than buying a brand new vehicle. What we didn't know was that whoever had demo-ed the car had smoked in it. The dealership had done a pretty good job cleaning the inside so it wasn't noticeable when we bought the vehicle, but when the following summer months came, it was like that old smoke smell cooked out of every surface of the car. It took a long time for that smell to go away. Even now, I don't know if the smell is gone, or if I'm just used to it. I called my sales man and complained about the smell, saying he should've told us. He swore that he didn't know, and maybe he didn't, who knows.
Buying a used car can be a great way to save thousands of dollars, but you have to be diligent so that you know as much about the vehicle as possible. You may be saving thousands, but are also spending thousands.