You didn't take another very important unit into account: time and risk. Let's say you want to sell your used iPhone. You might get $150, if you wait around for the right buyer, but this might take a while and even after waiting it is not guaranteed you find a buyer for that price. The price could even go down to $80. Alternatively you could go to a pawnshop and get $100 dollars instantly. And if you are happy with that price you can go on with your life and focus on other things.