Unfortunately no matter how much we love our cars and rant and rave about how much we've lost they simply are not worth as much as we would like. My 3 month old £31k S3 is probably worth £24k if I'm lucky. When you buy a car for £30K and slap 5 grand of options on it, those options are for you and not to add value. Yes your car is more desirable but it is not worth more money. It's still a £30K car minus vat and depreciation. Yes the dealer may give you more, but that is simply discount on a new car or he's hoping it can make it back somewhere else. And yes he will now stick your lovely car that you put loads of options on and sold cheap at a stupidly high price, to help make up for the next punter that want's loads of money for their car they are bringing in.
This is fundamental to buying and selling cars and one of the reasons I don't do it anymore because people just don't get it and end up being in a mood with you.
This still goes out to the Muppet in a Corsa who wanted to knock my head off because I told you your car would be worth more if you returned it back to stock and got rid of the wheels, big spoiler, lowered springs and decals down the side.