Many iPhone vendors don’t know this, and those who know it won't tell you.
Before you buy any fairly used iPhone at all, there’s a hidden Apple diagnostics trick that can help you know if the phone is in good condition.
Here's how:
1. Switch off the iPhone completely.
2. Hold both Volume Up + Volume Down buttons simultaneously.
3. Plug your charger while still holding the volume buttons.
4. Wait a few seconds while still holding the buttons.
5. The diagnostics page should appear after a while.
From there, you can test things like battery, Face ID, camera, speakers, sensors, etc.
Some supposedly “clean” iPhones already have hidden hardware faults. This process can help you spot them before you pay.
You can also check this on an iPhone you're already using, so you can know if your phone is all right.
There's also an app that can help you do this.
Maybe I'll talk about it later in another post.