> First, the car is worth whatever someone will pay for it
This is a nice little definition except that it doesn't mean much in the real world. OP could have gotten a better price if he invested some time and sold the car himself. He could have gotten a price closer to his own perceived value of it's worth.
Actually, it means exactly that in the "real world." All you're stating is that he could spend time in order to make someone else's perceived value of it closer to his own.
It doesn't matter what you paid or what the blue book value is. You will only ever get what the person willing to pay the most for it that you're able to sell it to will pay. I didn't like it or agree with it the first time I heard it (from someone who ended up not buying a car I was trying to sell) but, years and many experiences later, I understand that it's true.
If he invested some time then he would not just be selling the car as-is where-is. He would be selling the product of the car + his effort which may actually be a net loss depending on how much time he spent and how much of a better price he received.
If I had spent 2 minutes posting the photos and a description of the car, responded to 20 emails @ 1 minute each, then spent maybe 2-3 10 minute visits with people to sell the car it would've been a net win even if I did that every day for a week.
This is a nice little definition except that it doesn't mean much in the real world. OP could have gotten a better price if he invested some time and sold the car himself. He could have gotten a price closer to his own perceived value of it's worth.