When buying a second hand car it is important to try and establish its history and if possible have a mechanic give it the once over before agreeing to buy it. Unfortunately many people sell second hand cars knowing that there is a fault with it that may be intermittent and therefore they do not tell the new owner.
Many companies such as the AA over an inspection service where you can pay for them to come out and look over a car before you buy it. They will tell you if they find any faults with the vehicle and check things such as tyres and brakes for wear and tear. Although this will sot you extra, it could mean you avoid buying a car that requires a lot of money to be spent on it.
This will not guarantee that there are no faults but should pick up on most major ones that are likely to cost the most to repair.