The only advice I would offer is to cost out all these jobs which have appeared and decide whether its better to offer the car at top book with all these jobs done, or if its more cost-effective to sell the car at a price which takes the outstanding costs which await the next owner into account.
Unlike a trader, your only obligation is to describe a vehicle accurately, and sell a roadworthy vehicle. Punters will expect to pay less, so for example;
say the car (with £500 spent on it) should
be worth £3800, versus selling it at £3400. A car is only worth what someone will pay, and I would take the point of view that there are more people out there with the lesser amount to spend - so you may get not only more interest, but possibly not have to chip anything off the asking price.
Having detailed estimates on hand for any of the outstanding work, will prevent punters from overestimating the costs they will face (and trying to discount the car any further). Plus it saves you from investing money that you may not get anything back on. You can of course give the customer the option (once they have commited to buy), to pay the higher amount, and get the work done before they get the car.
Hope that helps,