Backup All Your Phone Data
The first thing you should do before selling your smartphone is to back it up. Sure, you don't want to lose some important data that you had on your old handset. You may copy all your data to your PC by connecting it through a USB cable or you may upload all the files to a cloud service like Dropbox, Google Drive, and One-Drive. The easiest way to back up your Android phone data is to use your Google Drive account. You can back up everything like photos, app data, bookmarks, calendar, contacts, and even your Wi-Fi network settings. To do it, head to Accounts -> Google Account and select all the data you want to sync for backup.
Factory Reset
After backing up all the important data, you need to wipe them off. This to prevent its new owner from getting your important data. Factory resetting your device will erase all data on your phone, including photos, music, documents, installed apps, contacts, calendar, and messages. To do this, go to Backup & reset Factory data reset.
Again Factory Reset your device after storing junk data (most important)
After Factory Reset you may thing your data was permanently deleted, actually it's not. After format any device like MicroSD, flash drive, HDD or smartphone all data will be hidden. Anyone can access your important data by using some powerful recovery software. So, after Factory Reset you need your device to start without any account info. Then full your device with junk data. After filling junk data your device will erase your previous hidden data and new data will take it's place. Then again Factory Reset your device.
Remove Memory Card
Don't forget to remove your memory card. You're probably going to want to continue using it on your new device. If you do plan to give away your microSD card, be sure to back up the contents and thoroughly wipe the card. You can erase your SD card by navigating to Settings -> Storage, then tap Format SD card.
Clean It
This is a small thing, but it's quite important. Cleaning your device can help you sell your device at a better price. Wiping of the screen, removing any dust, and cleaning inside the ports are great ways to keep your device clean and looking brand new.
