You can buy any iPhone but first, make sure that he/she is not selling you a locked iPhone.

How To check?

Turn on the device

If its ask for the passcode, face ID, or fingerprint the device has not been reset back to factory defaults
Ask the seller to do the following before you pay them:

Unlock the device
Click Settings > General > Reset > Erase All Content and Settings
They will be asked for the Apple ID and password to do this

If they can’t unlock the device then it's not good for you.

But if you already bought a locked iPhone then don't worry, you can unlock it by yourself by following the below article's guide.

https://www.ioscraze.com/how-to-bypass-icloud-activation-lock/