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You are going to want to do a factory reset to remove your accounts and information from the device. Just go into settings menu and find the option, which will vary slightly between devices and Android versions (settings>personal>device reset in Android 4.4). Don’t forget about your SD card, if you have one in the phone. Remove it and hang onto it, or at the very least wipe it so all those, ahem, personal photos don’t end up with someone else. Also, remember to remove your SIM card, if applicable. Beyond doing these things for your personal protection, try to clean the phone up and locate the original accessories and packaging if you still have it. Depending on where you sell it, this can make a difference in how much you get for it. Private buyers, more so than companies that take used phones, really like having all the boxes, paperwork, etc.