I’ve been selling cars for years.
Selling privately will get you a better price than ANY buy it now company.
Where is best to sell depends on the value of the car and type.
Cheap cars then Facebook market place.
5k plus then I find a classified advert on eBay works well and is cheap.
Autotrader adverts are expensive and don’t find I get as good a response as the eBay ad but are better if your car is 10k plus or premium brand.
But I would put all adverts regardless of price on Facebook and gumtree.