Many of us cannot afford to buy a new car and therefore have to buy second hand. Buying a second hand car is always a bit of a risk as you do not know its full history and often you have little or no warranty with it.
You can carry out a HPI check on the vehicle which I strongly recommend which will check to see if it has outstanding finance on it, has been written off or had plate changes. Most garages selling vehicle will automatically do this, but you can request to see the report if you wish.
Buying from a garage does give you a little bit more comeback than buying a second hand car privately as you should get a three month warranty as minimum from most reputable dealers.
If buying second hand privately then it may be worth investing in a used car check that can be done by companies such as the AA. They will come and give the car a once over before you buy to let you know of any issues they find.