A couple of things here - once I heard they were from MA and the truck passed inspection, she was even MORE out of gas than she was before. MM is NOT wrong about MA being way way (too) consumer friendly. I went through this once with a private party sale (I was the seller and there was nothing wrong with the car).
ALso - you will get less money from the dealer but the money applied to the new vehicle is NOT taxed, so you also have to consider that. If you sell privately and use as a down payment on the new car, you're charged sales tax on the WHOLE amount of the new car purchase. If you trade in, you're only taxed on the new car amount LESS the trade in amount. Both what you mentioned and that tax issue is why I trade in.