How do I wipe my hard drive before selling my laptop?

Before you selling your Mac, you need to clean up Mac ,

Maybe some people say they can use the "Erase All Thing" function to format the Mac , but actually this method also doesn't 100% safe. With a professional data recovery software, your deleted data can come back with ease.

Click here and follow the instructions. If the computer originally shipped with Mac OS X 10.6.8 or earlier, when you reach step 5, connect its original thumbdrive, restart with the C or Option key held down, use the Disk Utility to erase the internal drive, and install a fresh OS.

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I suspect your Macbook Air has an SSD, and I think Disk Utility disables the erase with zeros and secure erase for an SSD.

I took the approach that I would encrypt my SSD via System Preferences -> Security -> FileVault so that all data is encrypted, such that when I pass the system along, all I need to do is install the OS clean, and all residual data will be encrypted gibberish without a decryption key.

I suppose you could install a clean copy of the OS, then get a huge file (maybe the OS installer) and keep duplicating it until the disk is full. While that disk would not be all zeros it would have data you do not care access it, as it is just stuff they can get on the internet and nothing of yours.