You have overwritten your phone once

Used phones may contain illegal data that could be recovered and analyzed. To avoid this becoming your downfall, you should completely overwrite the phone once. If you have the possibility, generate large random files and copy them to your phone until it is full. Otherwise, you can also download large test files from the Internet and overwrite your phone's memory with them.

Caution! Overwriting flash memory is often not 100% possible. Data can still be left behind. With more modern Android devices and iPhones, this is usually not necessary due to the encrypted file system. In this case, however, make sure that the phone has been properly reset to factory settings. If you are not sure, you can still overwrite it.