How long does it take to do an apple ID unlock on a phone that you purchase
How long does it take to remove an Apple ID from a phone that you purchase from a second vendor?
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If the vendor went thru these steps, it should be ready for you to set it up on your own account.
Sell, give away, or trade in your iPhone - Apple Support
If they did not do this, ask them to do it or give you your money back. You will not be able to use that iPhone.