Here's the inside scoop... KBB and Edmunds do not loan money, to anyone. They are websites designed to drive web traffic for advertising revenues. Therefore, neither are reliable or accurate. However, both mentioned websites will get you within a dollar or so of the original dealer's cost; before any dealer rebates or dealer volume rebates.
To know a vehicles true worth, one would need to know what that exact vehicle (make, model, year, miles) is going for at a local auction and how many of those vehicles are for sell in a specific area. Car values are supply and demand based, like virtually everything else in the world. Dealerships are privy to this auction information, as well as NADA black book wholesale values. The appraiser at the dealership is in the best position (assuming they are being straight with you) to let you know what your car is worth.
The bottom line to getting the best car deal is knowing what you want in your next vehicle and having the means to afford it. If you are buying new, shop the other dealers in the area and then go with the salesperson who is working hard to earn your business. These will be the same folks you rely on when/if there are any issues moving forward. If buying pre-owned, have a budget in mind BEFORE you go to the dealership. If not, you can end up paying way more than you'd have to. Be honest and upfront with your salesperson about your intentions, timeframe, and budget. Any sales professional worth their desk and chair will appreciate it and most likely be willing to negotiate on your behalf if need be.
Dealerships, by most accords, have changed drastically because of the internet, especially on new vehicles. Everyone pays the same for the vehicles they sell, however the higher volume dealerships are usually in a better position to negotiate a little on the price.
Hope this helps...