Q: How do I wipe my iMac before a sale?

Hello,

I am selling my 2012 iMac so I can more easily afford a new Macbook Pro.

In order to sell it, I need to feel confident that I have permanently rid it of all my personal and business information and documents. Everything has been backed up to external hard drives.

My main question: what exactly do I need to do in order to effectively wipe the computer?

One of the first steps I've tried taking is to erase the free free space on my drive.  I do it with the following command:

diskutil secureErase freespace LEVEL (2 for a 7-pass erase) /Volumes/DRIVENAME

Every time I run this, it finishes up with an error hours and hours later, suggesting no temp file/folder could be created.

Thanks in advance for your input and insights.