Selling a car is difficult if you have no idea of what the cars history consists of. An easy way to ensure you can give the buyer all of the information they require in order to make an informed decision is to keep a ledger or log book up to date with the work that you have had done to it. Starting with the actual sale itself the buyer will know when they need to get the brakes done or when they will need to change the timing belt. Most cars require a timing belt change every couple of hundred thousand kilometres. If they dont know that it has been done the last time it has needed it you are putting them in a difficult situation.
If the car doesnt have a log book or ledger then make one. You dont need to go overboard on the details for each entry. Just write a couple of sentences about the work done and the date that it was done. For extra measure include the receipt. Keeping the receipt is important so you have some sort of proof in case the work that was done turns out to do more harm than good.
Selling a car has a lot to do with information sharing. The more information you can give the buyer the better. The will probably thank you for it in the long run too.
Certain things (besides the fore mentioned timing belt) need to be done at set intervals. It is actually pretty hard to determine when you need an oil change if you are using darker coloured oil. Selling a car that the buyer has a clear indication of what needs to be done when will help the buyer keep the car in good condition so when they want to sell it they can be confident that they have maintained it adequately. For the few minutes it takes to update the log book each time work has been done to the car you can potentially save thousands in the amount that you can be knocked down without having some evidence to back up your claims during price negotiations. It is such a simple thing that can save not only your buyers peace of mind but your own.