How do you determine if a car is priced too high or too low?

I look at the same make, model, condition and year of similar cars on Craigslist, Autotrader, dealer websites and ebay. I gauge the average value for about 10 of them. I find that places like KBB are often way out on pricing, up to 50%. eg I was looking for a 2008 Porsche Cayenne Turbo a year ago, and they valued that model at $25k less than the cheapest one in the United States. You couldn't even find a wrecked, salvage title car at the price they were quoting.