Rule one.
Cars of any kind only have one price, that's what the seller is willing to take and the buyer is willing to pay.
A bit more complex than that, and on-topic for this thread and anyone buying/selling cars, or anything else for that matter.
There is a maximum that the buyer will pay and a minimum the seller will accept. If these match then there is a single agreed price, but more usually they either don't meet, in which case there is no sale, or there is an overlap in which case there is a window in which a deal can be agreed. The trick is to identify where that window is.
It is actually a bit more subtle than a simple window. At the upper end there is usually a price which the buyer will pay but not be happy about; at the lower end is a price the seller will accept but not be happy about. Whether you are aiming to be in the central region where both sides are happy, or at one end or the other, depends a lot on whether you have a relationship between the buyer and the seller. If there is a relationship you need everyone to be happy, otherwise the relationship will be damaged which may be more important than the short-term financial outcome.
Usually, the need for a relationship depends on the need for repeat business. If it is a one-off purchase with little likelihood of repeat business then there is no need for a relationship and both sides will do best by fighting very hard. This is one of the reasons that estate agents and used car salesmen have such a bad reputation - they will probably have moved on by the next time you buy a house or car so have nothing to gain from a good relationship. By contrast, your garage/barber/car parts shop has every reason to want your long term custom and you don't want to have to keep shopping around, so it is easier for everyone to reach a figure that both parties are genuinely happy with.
The other thing not to forget about is identifying other aspects of a sale/purchase which may be of negligible value to you, but very significant to the other party (or vice versa). For example, how long can they hold it for you? do they have a mate who will drop it off for peanuts? is there a garage full of spares which, once the vehicle has gone will be useless and actually cost them time and effort taking them to the tip? Questions like this should be discussed before the price as they define the specific value, to you and the other party, at that point in time. A non-car related example - when we bought this house there was a 40ft storage container in the garden which the owners were going to have to empty anyway. We were moving about 10 miles so we agreed that, on exchange of contracts, they would make it available to us. I moved all my garage and outdoor stuff first, then all the small indoor stuff and finally the weekend before moving borrowed work's van and moved the furniture. Spreading it over the 3 weeks between exchange and completion saved me £3k in moving costs and cost the previous owner nothing.
Alec