To do this, you have to know the market. For example, what is the average asking price for this car? In the present time, you can easily answer this question by using the internet. Make sure, however, that you compare equal cars with comparable mileage, kit and production date. This way you will never buy an overpriced car.
Of course, there are organizations where you can request buying and selling prices. Often you have to pay for this information. Note that this information is not aware of the general condition of a vehicle, nor of the difference in trade prices given by brand- and non-brand dealerships. In short, you can use these prices as a guideline, but in most cases the outcome will be wrong. You can better do your own research based on real world prices and calculate the average asking price yourself.
Trade price
Are you absolutely sure that the car is in good condition, then most dealerships will give you the option to buy the car for the trade price. This means that you buy it without any guarantees and maintenance. Negotiate, however, at least a fresh MOT and three months warranty on the moving parts (for example, the engine, gearbox, differential, and drive shafts). This gives you the assurance that you can use your car without having to repair hidden defects on the vital and expensive parts.
This arrangement is especially good, when you buy a dealer's demonstration car or any other newly registered vehicle. Most cars have at least a two year factory warranty, so you do not need additional guarantees. In case of technical failure, however, you will have to take the initiative and contact the nearest brand dealership or the importer. Because you will lose a part of the dealer's service, it is always smart to contact a brand dealership or the importer beforehand. You want to find out what the extent of the service is in case of technical problems and for how many months you are still covered.