I have been involved in the buying/selling of many thousands of vehicles. In that process I learned 75% of units are average, 20% are junk, and 5% are exceptional.
The exceptional units have been obsessively maintained, have all the right equipment/colors, and are usually cleaner than when new. The professional people selling/buying know that you can throw the "book" away when dealing with an exceptional unit or a "junk" one.
There is a reason why dealers pay dearly for professionals to buy/sell their inventory rather than hiring a guy off the street for $10 an hour and handing him a value guide. There are a significant number of units that are worth 50% less than the book and a smaller percentage that are easily worth 50% more. Trying to buy a unit for your personal use and limiting one's self to some magic formula based on some mythical value guide is a fool's errand.