My advice: never trade your car in - you'll never get a good deal. Instead, do the graft yourself, market your vehicle privately, suffer a few deluded tyre kickers, and sell it for what you believe it's worth, or as close as. And if a private buyer offers you an insulting price, then it's your prerogative to tell them to do one.
If, on the other hand, you desperately want a new car/want a quick sale, then I'm afraid you've got little choice other than to bend over, lube up and accept you're being shafted.