It is a great idea to buy a second hand car if you want to save yourself a bit of money. Cars lose, on average, 40% of their value in the first year and so buying a car after this time will allow you to get the best bargain. 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.
For this reason you need to try and protect yourself as much as you can by carrying out a few of your own checks. Always carry out a HPI check on the vehicle unless the garage have dome one and can show you the recent report. The HPI will check to see if it has outstanding finance on it, has been written off or had plate changes along with other information which can help you get a background of the vehicle. Most garages selling vehicle will automatically do this, but you can request to see the report if you wish. If the garage does not want to show you or will not do a report then I would see this as very suspicious and would not want to buy from them.
Buying from a garage does give you a little bit more comeback than buying a second hand car privately as often you will get some sort of warranty, but the warranty is often quite limited.