Icloud lock / 2nd hand device

If a used iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it).

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you, nor find the previous owner for you, and neither can the websites that offer to unlock it for a fee: these are scams.

You should if possible return it to wherever you bought it and ask for a refund as in this event the device is completely useless.

Without knowing the Apple ID and password , and activation lock is turned as on you can't operate the device see this article https://support.apple.com/en-in/HT201365

Read this https://www.apple.com/shop/trade-in

You'd have all the documentation showing the legit purchase,

There really is no such thing.

A device locked by the Apple ID john@icloud.com is considered to be the property of that account holder by Apple.

The only exception is when you have the original bill of sale since you are the original owner.