I have erased my Mac prior to selling but now I can’t get OS X to install please help

Depending on Age of iMac, it may work from Internal Recovery Mode. Shutdown and disconnect any external drives. Connect to router via Ethernet cable. Startup and immediately hold the three key all at the same time Command + Option + r. It will connect to Apple Servers and a Globe will appear slowly if internet connection and Servers are slow. Once it loads and presented with options, choose Install macOS. It will offer the most recent available version Compatible with this machine. Once it is finished and restarts with successful installation - it is done. When Set-up Assist appears just power down computer via Power button.

Since the machine is going to be Sold, No issue regarding signing out of iCloud and Find my Mac as long as when re-installing El Capitan no user account was created. If Setup Assist was never completed - your User ID is fine.

Now, if it is desired to remove this device from any association to your User ID - go to Here at Apple called Manage My ID. Log In using your Apple ID and Password and look for this specific Apple Product iMac 2012 and click Remove. Now, this device is no longer associated to your ID.