I check all three major book value sources (NADA, Kelley Blue Book, and Edmunds). They aren't perfect, but they'll get you in the ballpark for a realistic price. I like that KBB gives you a range, which helps in figuring out the negotiating range. Edmunds usually gives the lowest number, so I'll typically use that as a starting point. You absolutely must determine the highest price you are willing to pay, accounting for options, mileage, and condition of the vehicle, before you walk on the lot. Pay absolutely no attention to the list price. Then, as many other folks have already noted up-thread, you must be willing to walk if they won't give you that price. Some dealers won't be able to meet your price because they paid too much for the car. In my experience, they'll simply say "sorry" and let you leave. If this happens a few times, your price is probably too low. But if you are anywhere near their bottom line price, they will move heaven and earth to get you to stay. This is when you really have to stick to your guns. You've got them on the hook, so don't give in to higher counter-offers, up-sells, etc.
Personally, I advise against trading in. You are pretty much guaranteed to pay more overall. Selling a used car private party is not that big of a deal. The buyer is responsible for paying taxes, registration fees, etc. I just recently sold a 2001 Hyundai Elantra 2 days after I listed it. It cost me $25 to list it on AutoTrader. I got my asking price, which was $600 over the estimated trade-in value. Again, the key is doing some research to figure out what it is worth, considering options and condition. Price it a few hundred over what you think you can get for it, then be willing to negotiate down to your bottom line price. Over-pricing is the big mistake that most people make when trying to sell a car private party. You turn people off and it sits on the market for a while.
