Re: Resale value
If you can't get a high enough price, you keep the car. The advertised retail prices for the GT are extremely high compared with genuine value, (which is auction price), and are just not being achieved. You can see the same cars advertised for months, and less than 1 in 10 on ebay results in a sale. Dealers have to advertise high so that residuals look better, but this can result in market stagnation.
I think your trade in offer is at least £2000 too low, depending on what you want to buy.