Apple - How do I restore a Mac to its original macOS version?
FIRST: Please read What to do before you sell, give away, or trade in your Mac on Apple's website.
As for your question, it doesn't state what model MacBook you're trying to sell, so it's not clear if Mac OS X El Capitan is the original OS it shipped with.
However, your best bet is to boot into macOS Recovery using the ShiftOption⌘R keyboard shortcut. This keyboard shortcut is designed to install the version of macOS that originally shipped with your Mac.*
To do this:
- Fully shut down your Mac
- Power up your Mac again but immediately press and hold the ShiftOption⌘R
- Keep the keys down until you see either an Apple logo or spinning globe appear on screen (Note: If you have a firmware password set on your system, then let go of the keys when the password prompt appears)
- Once the Utilities window appears you'll be in macOS Recovery Mode
Now choose to reinstall macOS and follow the prompts.
* For users of older Mac models this option will install the closest version of macOS still available to the original that shipped with it.
As recovery mode is not available on that Mac, your best bet might be to restore with whatever you can any way you can, Then...
Once the Mac is up and running, download a version of macOS that will run on that Mac. Just the installer, don't run it just leave it in /Applications. If it STARTS to install just quit the installer.
Then use a utility like DiskmakerX to create a bootable installer on an 8GB flash drive. There are other ways to make a bootable installer but this is still my favorite.
Once you have the bootable installer you can boot from the flash drive, wipe the drive of the MacBook and install macOS clean with no AppleID associated with it.
I've done this a few times and it works a treat.
To download and restore OS X, your computer's eligibility will be verified with Apple when installing older versions of Mac OS X for me every time meant only one: The certificate which is being used in the OS has expired. This will be correct for every OS later in the future (Sierra, High Sierra, etc).
The workaround is that simple: just change the date to when OS was released or just a bit later (for instance on the date of the last update for this version).
You don't need Apple ID to install OS X (you may log in during the installation process but it's totally optional).