Some dealers could well do, yes. We all know their only purpose in life is to make us happy by buying our part ex at an excellent price and then selling it at a loss....
A dealer will take in a car at £7k, then spend money on it bringing it up to saleable condition, then service/mot perhaps, then add a warranty. Then he really ought to be making a profit so by the time he's spent all his time and money on it and paid the rent and wages. All you've done is taken the bottom book price and extrapolated it to the top resale price.That's not how it usually works in the real world. Look at ti the other way. He offers £9.5k as a trade in, and after negotiation gives £10k. Then he gives it a prep and puts it on the forecourt at £12.5k, and someone squeezes him to £12k and makes a deal at that. That scenario would give a £2k profit off which he'd need to take off any incidentals.