To prevent someone else from misusing your data, before selling it make sure you disconnect your phone from your cloud accounts and remove your SIM card. If you are still connected to one of your services, the merchant will not be able to refurbish your smartphone and will have to send it back to you.
To help you out here's a short checklist:
✔ ️ Remove the SIM card after checking that your contacts are all stored in the cloud or on the SIM card and not in the device's memory.
✔ ️ Sign out of your iCloud, Google or Samsung account, otherwise your smartphone cannot be refurbished. You can also sign out of your iCloud account remotely.
✔ ️ Erase data or remove the SD card
✔ ️ Unlock your phone for each operator
✔ ️ Restore to factory settings (to delete all personal data from the smartphone: photos, apps, ...)