How do I factory reset my iMac from wifi screen

When you restart your system you can enter into internet recovery using the startup key option Command (⌘)-R

Reference: 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 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 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.