Activation Lock

Then they should go to https://iforgot.apple.com to retrieve their ID and reset their password, so that they can sign out of the device. Failing that, they (not you (whould take the device, the original purchase documents, and some ID to an actual Apple Store and ask if they can unlock it (this assumes they bought it new - if they bought it used this will not work).

Failing all that they should take it back and refund your money as you will never be able to use it.

Then the previous owner uses Apple's iForgot service or contacts their Account Security team, or gives you a refund.

The only other way to have it unlocked is for its original owner to supply Apple with its purchase receipt.

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The device is useless. See if they can recover the ID/password.

Apple ID - How to find your Apple         An iCloud e-mail address is an Apple ID.

Apple ID - Find

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage

Apple ID – Forgot Password