At a guess (and without having looked at any adverts on either site you've mentioned) I would suspect people selling cars look upon their pride and joy as being better than any other car on the market, and make an inflated estimate of its worth based on how many times a month it gets polished. Generally, they see a car for sale at a dealers (let's say- £10000) and think their similar age and mileage car is worth just that. In effect, selling a car privately for the same as a dealers price is not gonna happen, as the buyer gets a car with absolutely no comeback at all from the seller. From a dealer you get the weight of the law, warranty, etc. which costs money. And of course the dealer sometimes likes to make money on the sale so he can buy food. Private sellers should be looking to sell 15 - 20% less than a dealers forecourt price.
Then there's the other type of seller who adds up all the cost of the mods he's added to the car since owning it, applies this figure to the price of the dealers car, and bases his asking price on the total. Unfortunately, it's not like that in the real world.
The sellers who list a car at a sensible price for a sale are the ones who sell quickly. Unfortunately, even then people say "Why's it so cheap?" and get all suspicious about it.
So, find out what trade in price you can get for it, and then anything above that is going to be a bonus.
