Restore Macbook to factory settings

http://www.macworld.co.uk/how-to/mac/how-reset-macbook-air-pro-imac-restore-your-mac-original-factory-settings-3494564/

If the Mac OS is too old, you need to update it to the latest version of OS X first before you can do the reset. There is no restore partition on older versions of the OS, so the instructions on booting to Recovery Mode (step 5) only work once it has been updated.