I've dealt with buyers who think my price should be lower based on NADA/KBB, and sellers who think their cars are worth way more than they are because of NADA/KBB.
It's very simple to negotiate with these people - do it just like real estate. Use comps (comparables). Look at the market in your area. Find cars in similar condition and base the value off those cars. You can almost always find things to show why your car is worth more or their car is worth less than the comps, and use that for negotiations.
My favorite example is KBB & the E36 BMW convertibles. KBB has them thousands above what the market supports. Some of these sellers list these 10-15 year old convertibles with 150,000 miles on them at 10,000+. They're smoking crack. The market is supporting maybe 6-7,000 for those cars right now, and they all think they have much more expensive cars. They wonder why their cars have been sitting with no real interest in them for 5 months - it's because the market isn't really what KBB/NADA thinks it should be.
Whenever people try to negotiate with me using KBB/NADA, I stop them and tell them I'm not interested in talking about that. The current market is all that matters. Another good thing to mention is that KBB/NADA is a *LAGGING* indicator. It shows where the market WAS, not IS, and that doesn't matter right now.