Earlier quoted context omitted.
Driving a car until the wheels fall off generally doesn't make financial sense. Keeping an old car on the road gets really expensive unless you have the space, time and skills to do all the work yourself. Then there is the opportunity cost of not having access to your car for days/weeks at a time while it is being repaired. Finally you have the stress of not being quite sure if your car will start when you turn the k…
Same with technology. If you aren't rich, selling around the time the next model comes out is actually a great way to avoid paying full price for your modern tech. With Apple stuff, waiting more than 1-2 cycles means you've missed the ability to recoup most of your costs AND you got to use an old device for an extra year... yay?
If the phone only lasted 1 more year, then the resale price of the phone would reflect that. Either the collective market of buyers lacks the information to evaluate this (doubt), or you got lucky finding the one buyer willing to overpay.
You also always pay full price, because when you buy a new phone (or anything), you always have the option to buy the used or keep using the old one. Cost is opportunity cost, which is defined as your choice minus your second best choice.