I always prefer to buy things from stores. I don't usually purchase things from friends and I never buy things from strangers. I have bought things on ebay like some stamp sets and other small things. There's just something more secure about going in to an establishment or buying from a well known website. I don't want to say I am mistrusting but you never know what you will get. I have friends who purchase all kinds of things from Facebook or Craigslist without a problem though. I guess I have heard way too many horror stories or watch too many court shows. I also learned that you shouldn't buy used cars from friends or family. If something goes wrong, it could potentially ruin that relationship. It's easier to trust a friend but sometimes it doesn't work out and money does strange things to people. I also don't like to borrow or loan money to people because you never know what might happen. There have been times I have loaned a little money to someone without the real expectation of paying it back. I try really hard to keep money out of friendships. So in my opinion buying a used car from a friend is a big no way in my book.
To see the other reasons not to buy a used car from a private party check out my post.