Mac Reset to Factory / No Password

Follow these directions: What to do before you sell, give away, or trade in your Mac - Apple Support  

It'll do what you want.

1. Start up from macOS Recovery: turn on your Mac, then press and hold Command (⌘) and R immediately until you see an Apple logo or other image. 
2. If asked, select a user you know the password for and enter their administrator password. 
3. From the utilities window, select Disk Utility and click Continue.
4. Select Macintosh HD in the sidebar of Disk Utility. Can't see Macintosh HD?
5. Click the Erase button in the toolbar, then enter the requested details:

• • Name: Macintosh HD
  • Format: APFS or Mac OS Extended (Journaled), as recommended by Disk Utility

1. Click Erase Volume Group. If you can't see this button, click Erase instead.
2. If asked, enter your Apple ID. Forgotten your Apple ID?
3. Optional: if you used Disk Utility previously to add internal volumes other than Macintosh HD, you can erase them individually using the same process. 
4. When you've finished, close Disk Utility to return to the utilities window.
5. If you want your Mac to start up again from the volume you've just erased, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions. Find out more about reinstalling macOS.

After this procedure, your mac is completely empty and there is no Macos installed on it. Therefore you Will need to install Macos following these instructions How to reinstall macOS – Apple Support (UK)