The Verge: Before you give your device away, delete your personal data: “About to buy the latest Samsung phone or save a bit of money on a Google PIxel 3A? Whether you plan to trade in your old Android phone for a discount on your new phone, sell it on eBay, give it away to a friend, or drop it off for recycling, you’re going to want to wipe it of all your data first by resetting it to factory conditions. Luckily, that’s pretty easy to do.
Before you start, be aware that these instructions are for a Pixel 3 XL running Android 9, but the process for most current Android phones should be pretty much the same. Needless to say, first make sure that all of your data is transferred to your new phone, backed up, or both. Once your old phone has reset, there’s no going back…”