Hi there and welcome
With the new software they made for iOS if the original owner of the iPhone did not sign out of the iPhone before the Logicboard was removed, then it is locked. This is also the case if it is stolen. It is best to avoid these if you can as you cannot use them unless the seller can sign into the iCloud account and Then the seller logs into iCloud.com and goes to find my iPhone. Since it is powered off, it will not find it and the seller will have the option to remove it from find my iPhone. After that you can then use it, but you must make sure the Seller is capable of this, else its likely a stolen board. Apple will not unlock it for you.
Can you tell me please the problem you are having on the phone?
and thank you for the kind words
If you would like any additional information or assistance, please do not hesitate to let me know.
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