The best you can do is find what other cars similar to yours are selling for in your area. Problem is the dealer will give you wholesale value wich will be much lower than selling price.
The way to get the best deal is to sell your car yourself. I have used AutoTrader many times to sell cars. When you go to buy a new (or used car) you will get a much better deal when you do not have a trade in.
2008 Space Grey BMW 535XI Wagon
2006 Alpine White/Terra Cotta BMW 330xi
2000 Red Crownline 180BR - Lil Red
2003 Dark Blue Toyota 4Runner V8