These types of "unlocked" phones are only unlocked until you choose a carrier and use it with them. Then it is locked to that carrier as if you had purchased a locked phone to begin with.
If you want a used phone, then one of the very few trusted websites to get one from is Apple's own refurbished page:
https://www.apple.com/shop/refurbished/iphone
Unlike other, um, refurbished phones, Apple does whatever's necessary to make the phone look and function like new. Including replacing the screen or case. A new battery is always installed. They also carry a warranty, which you won't get from any other seller.
The only other trusted site I could recommend is Gazelle.
https://buy.gazelle.com/collections/iphones
They buy used devices and ensure they're in working order. They also confirm they are not, and will not end up in activation lock. Problem is, their prices really aren't much different than getting a refurbished phone from Apple. And the condition you get them from Gazelle is how they got them. You pay the most for the cleanest, least scratched ones they have. Less for beat up ones. What they have on hand varies with what they got from sellers.