When buying used Apple devices there are two things everyone should check for before paying.
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Activation lock has been removed. This just requires that the device is not logged in to an Apple ID.
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The device is not managed by a Mobile Management Profile. Schools and companies use these to manage the devices of students and/or employees to restrict what they can do with the devices.
I’ve never heard of anyone using activation lock as a sort of escrow to keep the device locked until they confirm payment.
Personally, I would never buy an Apple device from someone unless it is in person so I could verify the two things I pointed out above have been removed.