1. Download a copy of the Sierra installer. DON'T run it yet.
2. Use a USB flash drive 16gb (or larger) and ONE of the following:
- Boot Buddy
- DiskMaker X
- Install Disk Creator
...to create bootable USB flash drive with the installer on it, but... DON'T run it yet.
3. While still booted up in your current account, use Filevault to encrypt the entire drive (if you haven't done that already). This may take a little time, as mentioned above. When the drive has been encrypted, shut it down for the last time (you are about to erase it).
4. Boot from the USB flash drive installer.
5. Go to Disk Utility FIRST, and erase the internal drive. Set it up for HFS+ with journaling enabled. Do this, and your prior data will be unrecoverable (IF you used Filevault on the drive first)
6. NOW start the installer and do a fresh install. When done, it should boot to the "brand new Mac" screen, asking you to begin setup. At this point, I would just "pull the plug" and let it go dark. Leave it to the new owner to proceed from here.