What to check when I buy a used iPhone?

Be very careful about buying a used iPhone.

I don't know of a way for you to to check hardware condition – beyond what you can do by inspection, and looking at the Battery Health. However, if you're not careful, you might wind up with an Activation Locked, possibly-stolen "brick"; a carrier-locked phone; etc.

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community

If you want to buy a preowned iPhone - Apple Support

Someone who is selling their iPhone should follow the procedures outlined here. Note that these procedures will NOT clear a carrier lock. If a phone is carrier-locked, only the carrier t whom the phone is locked can remove the lock, and they often will do so only for the person whom originally had service with them.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If the online Apple Store in your country has a Certified Refurbished section, you may want to consider looking for a phone there.

The discounts are usually only about 15% off original list price, but Apple's refurbishment process is very thorough ("All refurbished iPhone models come with a new battery, a new outer shell, a one-year limited warranty, and free delivery and returns.").

Refurbished iPhone Deals - Apple

We see multiple posts each day from users who regret buying a used phone.

Before you buy.......make sure that you have a written agreement that will allow you to return the phone for a full refund within 7-10 days.

The reason......there are things that you cannot know about the phone until you physically possess the phone and try to set it up.