how to return to factory settings

First of all, save all the files you want to keep (photos, documents etc.) on a separate drive or in the cloud. Make sure you are connected to the Internet. After that, follow these steps:

1. Restart your Mac. Once it restarts and the grey screen appears, press and hold the Command and R keys.
2. Select Disk Utility and click Continue.
3. Select your startup disk on the left and click the Erase tab.
4. Choose Mac OS Extended (Journaled) from the Format menu, enter a name, and then click Erase.
5. Select Reinstall OS X, click Continue, and then follow the instructions on your screen

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

Apples knowledgebase on what to do before selling your mac is essentially restoring it to a out-of-the-box state.

What to do before selling or giving away your Mac - Apple Support