Well, that's why I would prefer to ignore profiteers and sell privately. But which private selling platform is the most efficient?

I'm not sure it counts as a "selling platform", but don't forget the free (or nearly free) adverts you can put on the noticeboard in your local Co-Op, Tesco, newsagents etc.

A lot of people are suspicious of buying a £9k car privately though, so the best price may well come from one of those car buying places anyway. As a private seller, people will be expecting the car to be quite a bit cheaper than one of the same age/mileage from a dealer where it would come with some kind of warranty and comeback if it did go wrong.

Everyone I know who's privately advertised a car worth a reasonable amount of money on Autotrader recently has ended up selling it to a trader who is looking for stock rather than someone who is intending to use it themselves. Obviously nothing wrong with that as their cash is as good as anyone elses. The likes of Motorway, Cazoo etc are really just another way for those same traders to get stock rather than trawling through Autotrader looking for decent quality stock, so you don't necessarily get a better price selling to them via the old fashioned route.

Also, with the shortage of stock at the moment, if the car is in good condition and had a service history. it can also be worth approaching any established local secondhand dealers to ask if they are looking to buy stock. If you car is what they want, cutting out the middleman is beneficial for both parties so that can get a better price too.