I've never traded in a car and I never will. This last car I wanted to sell quick, and not mess with. It had some mechanical issues and, since I was already planning on getting a new car next year, I just went ahead and bought a new car THIS year and just decided to get rid of that one. Just gave it a quick vaccuming of the interior and washed it. And asked about $1000 more than the trade in offer (I told the dealer I didn't wanna trade but the salesman gave me the usual 'You might be surprised' spiel and I said whatever, appraise it; only because I was curious.), which was still well below book value. Cash was in my hand and the car was gone that afternoon.
These days, selling a car is EASY and it's free money. If I had been motivated, I could've slapped a few hundred more on TOP of that and probably gotten it with the right buyer. Could've also given it a proper detail and fixed the mechanical issues and gotten top dollar for it. But, the fact that I got an extra thousand bucks for very, very little work in the scope of one afternoon? Why would you ever trade in!