When it's from a stranger, ie Craig's List or AutoTrader. It takes a lot of due diligence, but you can find a good one that's way cheaper than a dealership.
  1. Get a Carfax membership
    It's $54.99 and good for two months. You can search 5 cars by VIN and unlimited by plate number. Since you're buying privately, plates will usually be registered. If a seller doesn't want to share plate info until you get there, it's probably going to be a salvaged title or have a bad history.
  2. If you don't know the area, or feel unsafe, meet in a public location.
    Grocery store parking lots are usually best, since there is ample parking, visibility, and easy to find.
  3. Try meeting near a freeway or a major road with a high speed limit.
    Test driving at 30mph provides some insight into basic car function, but taking it north of 60mph will show you how well it's been taken care of, if it needs an alignment, etc.
  4. Try purchasing from a woman, especially a mother, if you can.
    Men fit the worst male stereotypes when it comes to selling their cars. You can see their dongs shrinking when they take any money off, like it hurts them. I've purchased 5 used cars privately and they've all been with women. Why? They take care of their cars and never try to cut corners when buying parts or maintenance. Moms especially, since they don't want their kids to die in the car. AND they usually price the car fairly, so there's minimal haggling to be done.
  5. If you live in CA or a state that requires intense smog checks, it is usually the seller's responsibility
    If it won't pass, they should discount the car... if in California, the discount should be substantial.
  6. If you can't bring a mechanic with you, plan ahead.
    If it's close enough, have the seller meet you there or schedule a time. If it's too far, google reputable shops, schedule an appt, or just take it to the closest Sears/Pep Boys/Firestone. They'll do a basic inspection for free or $20 and a more thorough one for like $50 or $90.
  7. Negotiating
    Do your homework. Check KBB and then see what it's going for in your area. Fair market value is usually a bit more than KBB, depending on the condition, care of the car, etc. Carfax has an algorithm that gives you a more accurate rate for your area. They will usually come down on the price. If the mechanic finds major issues, that'll work in your favor. There's only so much that people can fix, so things like fluids and tires you can't put on others, but major parts and issues you can.
  8. Good luck!