If you plan to sell mobile phone online or to a local buyer, protecting your personal data should be your first priority. Before handing over your device, back up important photos, contacts, and files, then sign out of your Google or Apple account and remove any linked services. Disable screen locks, Find My iPhone, or Find My Device where applicable, and remove your SIM card and memory card. Finally, perform a complete factory reset to erase your personal information and restore the phone to its original setup state. Taking these steps helps prevent unauthorized access and ensures the buyer can activate and use the device without account-related issues.