If you are buying a new or second hand car from a garage then you may be able to part exchange your old car with them towards the cost of the new one. When a garage takes in a vehicle in part exchange they will then decide if they are going to try and retail it or if they are going to send it off to auction. If the car is not worth much then the chances are it will be sent to auction.
If the car is likely to be retailed then the garage will want to take it in in as good a condition as possible and so they have to spend less money on getting it ready for sale. Making sure the car has a long MOT, is clean inside and out and has been looked after in terms of services and parts like tyre and brakes, will often get you a little bit more for your part exchange, but before splashing out on MOT’s etc always ask the garage if they will give you any more for it with a fresh MOT as they may not be able to.
Garages often go mostly on age and mileage of the vehicle also taking in to account the body work. They may not even want to know about work you have had done on the car but may be interested in the service history and MOT due date.