How to Unsync my MacBooks before I restore factory settings on one of them?

I'm selling my old MacBook and want to make sure I don't lose files on my current one during the process of deleting files and restoring factory settings. I'm deleting apps and files from my old MacBook to see how fast it is before selling. How can I unsync my Macs so I can do this without compromising files and apps on my new laptop?