Just this week, I helped my father sell his low mileage Honda CR-V, as his doctor ordered him to stop driving.
KBB says trade-in value was about $9k. NADA said more like $7000. Edmunds was even more pessimistic at $6k.
CarMax offered $9k. It sounded great, so we tried our luck with a nearby Honda dealer. They were only willing to offer $8500, but after showing them CarMax's written offer, and what other car dealers wanted for a similar vehicle* we finally got them to $9500 and finalized the deal.
So in this case, KBB was actually the closest, and slightly undervalued the car.
*Used car dealers in the area were selling the same vehicle for $10k with far greater miles on them. CarMax wanted $12-13k for vehicles with double or triple the mileage in some cases. So I knew that we could get a decent amount for my father's low mileage car.