Whether selling your old motor yourself, or part-exchanging, do some homework first, says James Ruppert
There, gleaming in the showroom, is the car of your dreams. The trouble is, parked outside stands your old nightmare of a motor and it must be sold before you can afford the new car. And selling a secondhand car is often not easy.

Of course, the car salesman is all too willing to help out by inviting you to part-exchange. Now that sounds like a sensible and simple solution, but have you ever stopped to consider what is in it for the dealers? Before you take the part-ex plunge, there are a few things you really ought to know.

What every private car owner should understand is that a car dealer will offer you only the trade value of your vehicle. Nothing more, and sometimes less. This is effectively the wholesale price for a particular model. Adjustments are made for condition, age and mileage, and there will be furtive glances into the car trade price bibles (Glass's Guide and CAP Black Book) to confirm that figure. However, you will probably never know exactly what that sum is.

Visit three different dealers and offer your car as a part-exchange against an identical model, and the chances are you will be offered three differing figures. What happens is that the salesman uses some of the profit from the new car you are buying to inflate the trade-in value of your old car. Often that is enough to tempt someone to part-exchange on the spot. But the customer loses out because negotiating a discount on the new car is not on.

You can win at the part-exchange game, though, by doing a little research and uttering the magic words "price to change". All you need to discover is the true retail price of the new car and the real trade value of the part-exchange: the difference between the two is the price you want to pay.

Buy a price guide from your newsagent. These are small-format magazines and are the public's equivalent of the trade guides. They will reveal how much your car is worth in trade terms, which is what you will effectively receive. To double-check this figure (and for a local perspective), ring up some dealers and offer to sell them your car - and see what figures they come up with. If the replacement car is secondhand, use the guide to calculate the difference between trade and retail price to guesstimate their profit. That is the figure you need to erode to get the best deal.

If you are buying a new car, then the best policy is to shop around and find out who offers the best "price to change", the difference between what you get for your old car and the price of the replacement. Sometimes it will pay to indicate you understand the game and ask: "how much will you really pay for my car and what's the best deal you'll do on the new one?" One of the best tactics is to negotiate on the new car without mentioning a part-exchange, secure a good discount, then at the last moment introduce your old car. Now that the salesman has invested his time, he will not want to retreat. That way you should preserve the discount on the new car and get a realistic trade price on your old one.

Beware the salesman who cannot quite do the deal on the model you want, but will on something else. That something else could be overpriced in the first place, an old model that has recently been replaced, or a deeply unpopular car which will depreciate fast. Do not be fobbed off until you have done some research and do judge each deal on its merits.

When you see a "minimum pounds l,000 part-exchange deal", even if your car is worth pounds 25 scrap, dealers will still pay a four-figure sum. They are not being over-generous, but simply offering an incentive to buy a new car. This has been a popular tactic to help sell bargain basement Eastern bloc cars and effectively it means there is at least a pounds l,000 discount available. So life would be easier if you breezed in without a car and knocked even more off the price.

If you do not part-exchange, the alternative is to sell your old car first. It is by far the best way to get a decent motoring deal, as you can go cash-in-hand to the dealer and negotiate from a position of strength. That way you do get more than the trade price, but you also get several times more hassle.

The best way to arrive at a fair retail price is to look at local advertisements to see what is being offered for similar cars. Decide on where to place your ad: cheap cars sell better in the local paper or free ads publications; specialist cars ought to be sold in relevant sections of the local Autotrader; and prestigious cars should find their way into the classifieds of national papers.

Write a brief and to-the-point ad: model, mileage, colour and price are the essentials. Now clean the car, or get a valet company to do it for you at pounds 30 to pounds 50. (Dirty cars do not sell and this is a good tip when part- exchanging, because even hard-hearted motor traders may pay a few pounds more for a tidy looking motor.) Gather together any information you have about the car - handbooks, MOT certificates, service histories - and put them in a file. This always creates a good impression.

Prepare for the worst. Do not let a potential buyer drive the car until you have proof that they are insured and have a licence. Never let them drive off on their own. Be polite, honest and prepared to drop the price by a token pounds 100 to do a deal. Never ever hand over the keys until the cheque has cleared, or the hard cash adds up.

In fact, the hassle of selling privately makes the brief uncertainty of part-exchanging look very attractive. Whatever you decide to do, unloading your old car is never going to be easy.

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