Generally, if a car salesman isn't pissed off at you following the sale, you are getting ripped off.
Yep, they are usually pretty pissed at my Dad.
Ideally, he takes a loss on the car he sells me
He is not really taking a loss if he's meeting a target that net makes him a profit(in some manner). You seem to argue that you can't extract such information. I'm arguing that you can.
The point is he might be making a loss to the business for some other goal (e.g. making a quarterly number that his boss will reward him for, costing the business even more), so making assumptions based on the value of the expected average market value of the car has a decent chance of setting the value higher than the value of the car to that salesman at that point in time.
Yep, they are usually pretty pissed at my Dad.
Ideally, he takes a loss on the car he sells me
He is not really taking a loss if he's meeting a target that net makes him a profit(in some manner). You seem to argue that you can't extract such information. I'm arguing that you can.