Feel free to buy any car from anyone. However, if you want to minimize the chance of getting a car that was not meticulously maintained and/or was not driven mildly, I think the above general advice will give you a decent shot.
There's no problem in buying something if you like it and know what your'e getting into. There's also no problem if you don't specifically care what you're getting into.
However, if you do care about what you're getting into it's probably a safe idea to see you do everything you can to find out as much as possible about the car's history, and then to select a car with as few spots in its history as possible.
If you know about something you can then decide for yourself if it matters to you or not. For example, if you know a part has been changed you can at least ask what happened and gauge from there if you are interested, if the seller seems honest, if the seller seems competent, or both, or neither.
The more you know the more educated your choices would be.
All this is just IMO and I don't want to come across as dogmatic (though I am very proud that whoever buys my car one day will be pretty well assured it's been maintained correctly). Of course even my well maintained car could fail the moment I sign it over to someone else - there's no foolproof way to know that and even brand new cars have problems. I'd just like to see as many people as possible be in as strong a buying position as possible. I'm sure there are better ways to evaluate a car; all this is just my two cents.
Look at it this way: some members here are very competent mechanics and have gone through several major parts (motors, gearboxes etc.). If I were to buy from one of them, I'd already know this and they'd just tell me anyway -- both the buyer and the seller are up-front with one another and know what's going on with the car. Other people haven't spent as much time underneath an Impreza with bleeding knuckles and parts all over the floor, so they could probably use some words of caution.