How are you able to upgrade every year for $100? On trade in plans, you're constantly paying monthly part of the 0% loan on the total price of the phone. You don't get that back. Resale of iphones have also taken a bit of a hit, I only got $450 for my wife's 6S 128gb and that's after being on sale for awhile on craigslist. Ebay has slightly higher prices but you're losing more on paypal/auction fees.
I'm not talking about the monthly installment. Im talking about the price to upgrade, which is tax and upgrade fee.
As for making payments vs. buying outright and selling later is personal preference. Looks like we both paid an equal amount to use our phones but you went through the hassle of selling to a private party. I paid $420 to use my iPhone 6s Plus for a year and then trade it in for a brand new one for free, just have to pay tax