How to remove activation lock if the previous owner can no longer be contacted or found?

The previous owner might have stolen it, and not all crimes are reported; generally, supplying the original purchase receipt is necessary to prove that it hasn't been, and people who bought iPhones from random sellers won't have those.

This is particularly likely if the seller offered an unusually cheap price for it. If it seems too good to be true, it probably is.

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You can't(unless the Apple ID involved is yours). The activation lock is an anti-theft measure.

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