If you are looking to buy a second hand car then you can choose to either buy one privately or from a garage. If you are buying a car from a garage then you can expect to pay more for it that you would if buying private. Often the reason for this is that you will get a warranty if you buy from a car garage as opposed to a private seller. You do need to check this though as may smaller independent car garages do not give a warranty or only a very short one and this can sometimes mean it is not worth paying the extra. Another reason as to why you may want to use a garage to buy your car is if they are able to offer you finance. Some finance companies will make certain stipulations such as the car must not be over 10 years old or not have over a certain number of miles on it. This can mean you have to spend more than you intended on spending to buy the vehicle.
IF you do decide to buy from a garage, check what the reviews are like for the garage and what sort of reputation they have as this is very important and should be considered when you are making your decision on who to use.