Read the series at http://www.thetruthaboutcars.com/201...negotiating-2/
That's part 4, but links to the first 3 parts are in the intro beneath the photo. Read them in order.
I've bought used cars both from dealers and private parties. Today the advertising medium of choice is craigslist, but it's the choice for both scammers and regular folks who are selling a car. Serious sellers include their phone number, and serious buyers call. If there is no number I include mine in the response e-mail. My first question is how the seller can prove they are the original owner and confirm that the vehicle is still registered in their name. I would limit my search to those vehicles. Try especially to buy from a seller who has a folder full of records and kept the records of their original transaction. Like Lang says in the articles I referenced above, you are buying a previous owner as much as you are buying a car.
Insist on an inspection. Find out the best place to get your make and model inspected and have a chat with them in advance. A specialist can be provide a wealth of information on variations within the model that tend to break or be expensive to repair.
Craigslist sellers can be (and should be) just as wary about buyers. Go out of your way to be the buyer who makes it easy to transact safely. Meet sellers in a public place during daylight hours. Suggest that the final transaction can be in the lobby of their bank so funds can be verified and you can conduct business in a safe environment.
If you can close a deal quickly without getting a loan in the way you will be in a strong negotiating position with most private sellers--their market is limited to people who can actually scare up the money whereas dealers can easily facilitate (and profit from) lending.
The cheapest (per mile) cars I've ever owned were bought used relatively young and with relatively high miles because those tend to be easy highway miles. If you don't anticipate driving less than average this works well because the car will have average miles or less when you sell it in a few years.
CPO is a tough call. Another alternative is to buy a vehicle that still has at least six months of factory warranty on it. Six months is probably enough to identify and correct any issues the car may have or dump it quickly if you made a bad buy. A car that has a transferable extended factory warranty can be a good deal as well.