Formatting a used iPhone?

No unfortunately for security purposes, Apple will not allow this device to be reset when it is still connected to the previous owners apple i.d. What you can do is go to ebay, and search, icloud removal service. Read the descriptions before you just jump and buy one, see how they are going to do it and what information they need. If you have at least the full email address on the phone, then you should be able to do something.

https://www.ebay.com/itm/iCloud-Unlock-i...

i wouldnt buy this one specifically because the ebay seller does not have that high of ratings but look for something like that and let me know how it works out for you. Hope that helps, thanks and have a good day!