NADA Guides doesn't have Private Party prices, but is based on real transactions between consumers and dealers, and you can expect PP to be somewhere in between. But you also have Cars.com, AutoTrader.com and CraigsList where you can just find lots of cars close to what you want to buy, and see what people are asking for them. That should give you a real world range of what you can expect to pay, with a little wiggle room left for negotiating a better price.