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.