If you are an inexperienced car buyer then it can be best to stick to registered dealers as this can offer you some protection. Dealers typically offer two to three months warranties so if you pick up any problems after you have bought the car you can return it to the garage. Warrantees typically cover problems with the engine, gears, clutch and brakes. Cosmetic damage such as chipped windscreens and broken side mirrors will not be covered in most cases but you may be able to claim for them on your insurance.
You should never buy a car privately or from a dealer without giving it a test drive. There are a whole host of potentially expensive problems that can only be identified when the car is running in normal road conditions such as poor brakes, a worn clutch or engine misfiring. You should drive the car for at least 10 to 15 minutes and take it on a few different types of roads to ensure you can work through all the gears and test acceleration and braking.
Always check the paperwork thoroughly before you buy any used cars and ensure the details match those of the car registration and engine chassis number. If the owner or dealer is unable to produce the right paperwork then walk away, as even if the car is legitimate you will have problems trying to insure and tax the car (and also when you come to sell it on) if you do not get the right paperwork from the very beginning.