If you own a Nintendo 3DS handheld gaming console and you want to sell it on to a shop, you probably wanting to restore the system to factory settings. Resetting your Nintendo 3DS console will revert all of the settings and changes you made to the system back to factory default. It will also clear all data on the Nintendo 3DS that is stored onto the internal memory. Resetting the Nintendo 3DS console cannot be undone once performed on the console and you can't retrieve the existing data that is lost during the factory reset. So it would be ideal for you to back up your important data, such as game saves and downloadable games onto a external memory card before performing the factory reset.
Instructions on How to Reset your Nintendo 3DS/XL to Factory Settings
1) Power on your Nintendo 3DS console by pressing the 'Power' button.
2) Now from the home menu go to 'System Settings'.
3) Then go to 'Other Settings' and select 'Page 4'.
4) Then finally select 'Format System Memory' and proceed with the factory reset.