I personally think the CPO a great selling point/deal closer, but not necessarily a reason to increase the price (from a private party standpoint).
The way I value cars usually works pretty accurately:
define the main characteristics that are most relevant to the car (year, engine, mileage, transmission, major options - nav, sport package, color, moonroof, htd seats, etc -, and mileage range 10k +/-). Then I run an Edmunds TMV (very accurate), and KBB private party (so so accurate), NADA (closer than KBB). Then I do a nationwide search for the very specific criteria of all the options for the car listed above on autotrader and cars.com. Then I look at ebay motors to see what the car is actually selling for and how many bids it received. I take all of the numbers and do a mean/median and find a few cars in my city to compare it to, to see how relevant that number is. If the local number is higher, the car is priced well and should sell fast. If it's lower, then I lower the price to be right in the middle of what's local.
I never want a car to sit and just wait for it to sell - there's carrying costs, insurance costs, opportunity costs, hassle, etc. I like to price a car well to sell quickly but not sell for less than I would get on a trade in also considering the tax credit tradeoff. Good luck!
