Hard Drive:
Secure erase the drive from recovery mode. Do a seven pass secure erase.
https://www.backblaze.com/blog/how-to-wipe-a-mac-hard-drive/
I know a single pass is enough. I prefer the seven pass. You just set it and let it run for a day or two. What's the harm? It won't take you any extra effort. You just set it and check back the next day. Unless you are in a rush to sell, why not?
I do this for two reasons. To ensure the data is gone. To ensure the hard drive functions under extended load. If it handles that. You can be reasonably assured you are selling a computer with a reliable hard drive.
Solid State Drive:
If on an SSD. Turn on FileVault. Let it encrypt the drive. Fill it up with random data, then repartition with a clean install. The encryption will have scrambled any files beyond recovery.