If you are in the market for a used car then like everyone, you will want to get the best deal possible.
Shopping for a used car can be a worrying experience as you often have no guarantee that the car is mechanically sound and have to try and do some investigating yourself. If you now w mechanic then you may wish to ask them to come along with you when you are looking at a car to give it a once over, but even the best mechanics cannot always know if there is something due to go wrong or a part that is damaged without stripping the engine down completely.
You have a choice when buying a used car to buy from a garage / dealer or privately. You tend to pay a little more by buying through a garage but often will get a 3, 6 or 12 months warranty (this may only cover certain parts though).