Most financial institutions use NADA values when determining the value of a vehicle. That would probably be the best value out there, though be advised on older cars it doesn't really apply. 

Your best bet is to research cars in your area and see what they are going for.

Edmunds seems to have realistic values, better than KBBs. I would check out KBB trade-in values and Edmunds private party values. Most importantly, go on your craigslist and see what they are worth. Or you could ask here on the forum and we would be glad to tell you our opinions on the value.

Back in the day when I worked at carmax, Edmunds trade value was close to what they would offer and they usually offered less.

Kbb and edmonds both kinda overestimate a cars value when it comes to buying one, so what I do is take the exact same car and see what they then value it if I were to "sell that car" typically that is the value that I abide by. But in today's market you can easily see a 15-20% mark up on used cars. If you know how to talk to private sellers it works in your favor as well. Good luck.