When you restart your system you can enter into internet recovery using the startup key option ***Command (⌘)-R***
Reference: [https://support.apple.com/en-us/102603|Mac startup key combinations]
Depending on your internet speed this can take quite awhile. I find setting up an external drive (a 32GB USB thumb drive) before hand or using a second Mac system speeds up the process and later becomes handy!
Download the OS installer file if you don’t already have it [https://support.apple.com/en-us/102662|How to download and install macOS], copy it to a freshly formatted (a Mac GUID and a Journaled file system) then follow this guide to convert it into a bootable OS installer [https://support.apple.com/en-us/101578|Create a bootable installer for macOS]
Do make sure to reformat the internal drive if you are reselling it so all of your data is not recoverable.