Coming from a long line of new and used car dealers (GM, Willies, etc.), if you trade in a well used car to buy another used car, you get at best "wholesale" value on your trade and pay near retail for the used car you want. They know you are in a car you no longer like and they know they have the car you want (you have just shown all of your cards). This is why the used car business is lucrative.
The same "well used" car will get you closer to book "trade-in" value on a new car deal since there is a lot of profit in a new car sale. (You are not really getting anything more than wholesale, it just looks that way)
The experts will tell you that you should always sell your car yourself, not tie it into any other car deal, and let the market dictate your price. As my grandfather use to say, "there is always someone who will want your car really bad . . . you just have to be patient."
The best way I have found to sell a car is to place it in the IWANNA or a local paper that also puts it in Cars.com. Lots of opportunity for your car to find the person who really wants it, and ALWAYS put a price with your ad.
I also think that posting it up here and on mud's main board is a good idea, especially if you are a mud regular.