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.
Following in from this, I've just came out of Construction Machinery Sales and we used to do it like this......
If my new digger was costing me £75k then I would work out how much I would have in his trade in. If I wanted his trade in to stand me at £30k then I would be asking for £45k to swap. Obviously 'On Paper' I would ask the guy £80k for the new one and show him £35k for his, just incase he dropped his trade in and wanted to buy outright then we would still make £5k profit. So his trade in stands us at £30k and we sell it for £35k with no trade in against it then we would have made £5k on the whole deal
Basically, what you get on paper means next to nothing, the hugely important figure is the price to swap. The best way to negotiate a deal I find is to get the best Straight Sale price that you can. Tell them it is a straight sale with no trade in and cash payment, no finance. This way they will remove part of the buffer they put in to make it look like they value your car at more than they want 'In It' and will also take out any finance subsidy they have built in. Once you feel as though you have them to the very bottom, chuck in your trade in and finance requirements then watch the salesman's face drop
Matt
