Take a Look at Battery Life
It’s hard to judge battery life when you’re buying an iPhone. A great looking phone could house a horrible battery that’s been run down by poor charging practices. How can you be sure it’s in good condition?
Well, the best way to check is in person. You can go to the iPhone’s Settings, then tap Battery, then Battery Health. This will give you a report on maximum capacity so you can easily see how worn down the battery is from its original state. If you’re buying online, you can’t really do this, which is why it’s important to look for sites that guarantee performance or run tests of their own when possible.
Otherwise, Apple can replace iPhone batteries for a fee, so you may want to include this fee as part of your calculations.