If you are looking to buy a second hand car from a dealership you should always try and haggle a bit on the price. Not many car garages will stick firmly to the price advertised in the window and if you do not ask for a discount you will probably not be offered one.
Before you go in with an offer you need to think carefully and sensibly about the car you want to buy. If it is an older vehicle and on the forecourt for less than £3000 the chances are you will not get as much off the price as if you were buying a car for £10000. If you have a vehicle to part exchange, they may give you more than its worth if buying a more expensive vehicle but less if you are buying something a lot cheaper.
Have a good look round the car, through the service history and last MOT. If there are things that you notice that will need doing on the car almost immediately then make a note of them. When you speak to a member of the sales team, go through the list of items that needs to be done and make them an offer based on this. Always go in a little bit lower than you expect to pay for the vehicle so you have room to meet them in the middle if they make a higher offer.