As a private car seller, it’s important to maximise the value of your old car before getting rid of it by ensuring everything works properly and the car looks good. Although this might be difficult if you have a shiny new one sitting in the driveway, hundreds or even thousands of pounds could be at stake. 

Before you think about listing your car for sale, have it detailed or do it yourself. A professional detail may cost quite a bit, but it can add so much more to the value of the car, particularly if it has lived outside. If you chose to do the clean yourself make sure you clean in all the hard to get places, such as door openings, boot openings, and the engine bay.

Never leave anything lying around the interior or boot as it looks untidy to a potential buyer, additionally loose items can rattle and bang around creating a poor impression on a test drive. 
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Wash and polish the exterior including the wheels, windows and all chrome or stainless trim. Make sure the residue of polish is cleaned off afterwards as you don’t want to give the impression that you’ve just cleaned it up to sell it.  It is also worth having the engine oil changed a week or two before selling your car as it will look clean if the buyer pulls the dip stick, and doing it a week or two before will mean it won't look too fresh on the day.

Make sure the windscreen washer fluid and radiator are topped up as well, as it will look like you have cared about your car. 

You need to collate all documentation you have about your car, especially the car's service record to show the potential buyer that you have had the car serviced regularly. 

If the car has any mechanical problems, have them fixed before the sale unless you are prepared to negotiate with a potential buyer. Also selling your car with a few months of tax is worth considering otherwise you’ll need to be prepared to negotiate on this as well.