The instruction from your trade-in notification e-mail says that you have to perform a factory data reset and remove all of your e-mail accounts from the phone but few customers misunderstands the instruction or maybe the words need to be rephrased. After the factory data reset, the phone will trigger a google factory reset protection and will lock the phone when set-up wizard is started. The user will think that all of the data were removed from the phone but that is not the case. The Samsung USA triage team who inspects the trade-in phone will reject the mailed phone and will send an email to the customers saying "You did not reset the phone or Locks were not disabled" resulting to trade in charge back.
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