Repair your iPhones before trade-in or Apple may give you nothing
One of the reasons iPhones cost as much as they do is Apple knows most people aren't paying full price for them. You're almost always trading in an old device or getting some kind of carrier discount, right? Well, if you're doing the former, you may want to keep your old iPhone in good shape.
MacWorld executive editor Michael Simon wrote a cautionary tale purporting that Apple would not accept a 256GB variant of the iPhone 15 Pro Max (which should have nearly the highest trade-in value of any existing iPhone) for trade-in because the back of the phone was cracked from a drop. As Simon pointed out, the cost of repairing this blemish would be only $29, but rather than deduct $29 from the trade-in value, Apple instead decided his iPhone was worthless during the trade-in process.
The big lesson here is to just pay the $29 (or whatever your repair fee happens to cost) if there's any kind of imperfection with your iPhone before trading it in for an iPhone 16. It's not ideal, but perhaps Apple is banking on people not wanting to go through the effort. Don't let Apple win.
Until you give the company several hundred dollars for a new phone, anyway.