NADA is a good source, though a few dealers use the Kelly Blue Book. The online NADA gives only retail vales but a local library or credit union probably has the NADA wholesale book available for public use.
I would offer somewhere around high wholesale in a private sale, unless it was a really high-demand model in prime condition. For that matter, I would offer the same to a dealer. Dealers invariably buy (or take trade-ins) at low wholesale, so a private seller is faced with that as the alternative to selling to you.
If you use the online NADA retail book, use the low retail as the uper limit of what you might offer. Subtract for any major repairs required, e.g. new tires, engine/transmission, appliance problems, etc.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL