+1 for the Autotrader tip. Unlike NADA and all the others, Autotrader takes into account market conditions and returns a value usually very close to what most dealerships will offer.
That said, there are several points to consider:
First, what you perceive to be "excellent" may be relative. Minor scratches, worn tires, dirty carpet, small dings in the doors, etc. may seem trite to you, but to restore your car to excellent may cost more than you think.
Also, most manufaturer dealerships (at least in my area) retail used cars below KBB retail and, believe it or not, there is not a "huge profit" in most used cars. Can't speak to your area, but in my area the used car market is every bit as competitive as the new car market. Reputable dealerships cannot afford to rip off customers on the trade or at retail.
Also, there are money factors at play which KBB does not factor in but NADA does. For example, if your car is out of warranty, the loan value the banks are willing to underwrite drops dramatically. If you have a 2007 car valued by KBB at $18K fair and $20K good, at one year out of warrany, there's an excellent chance that the banks will not loan more than $20K on the car. If it retails for $24K the next purchaser will have to put down a very large amount which greatly reduces the market for the car. Therefore the dealership needs to retail the car at a price close to the loan value, so they will likely offer you a trade in price less than the KBB "fair" price.
Finally, some dealerships will offer more for trades and offer less relief on the MSRP of the new car. Others offer greater relief on the MSRP of the new car and less on the trades. The bottom line is - does the total deal work for you? If it does, go for it. If not, go somewhere else. Don't get too hung up on the trade alone or the MSRP alone... they run in tandem. Get hung up on the bottom line.