The only problem with KBB (Cox Automotive) and Autotrader (Cox Media Group) is that they are industry sponsored sources presented by the same PR organization in favor of auto dealers.. By their very nature they keep the used prices elevated. Oh... NADA guides? Owned by J.D. Power. Another PR firm. Again, sponsored by and paid for by the auto industry itself. Oh well, someone has to pay the bills.
There is very little that can be called "objective" about the pricing in any of these things. But it works in favor of the auto dealers, especially those that are not OEM sponsored dealerships that buy vehicle at auctions to resell. It is not uncommon for "joe's used cars" to pick up vehicle at auction at prices that are significantly lower than "book" values. That means the dealers that brought the cars to auction knew they were not worth book value also.
A "good deal" is one that a person can live with. But a good deal should be like lawsuits, wherein a good settlement is one where both parties feel equally screwed. It is certain the many dealerships themselves do not follow the NADA, KBB, etc in terms of car value. The only way they give someone the "book" value in trade is if the vehicle the customer is buying is getting far more than what it is actually worth in the deal. It becomes a mind numbing cycle in favor of the dealerships. This is why I prefer to keep new car and used car negotiations separate. When they are combined, one will lose out in favor of the other and the dealer has two ways to buffalo the customer instead of one.
In terms of buying from a private owner, unless I know them personally and their vehicle, my offer will start at the lowest price a dealer would offer for the vehicle in trade. Now KBB and NADA might help a little bit there. One has to question the validity of KBB and NADA in this regard... why is it the car has one value if trading and another value if buying from a private source. It shows how things are skewed, and we have no real way fo knowing how skewed. The value of the car should be the same across the board.