Selling with Catalina - ?

The first thing you need to do is for all users in all accounts on the machine to log in to their accounts, and then go into System Preferences -> Internet Accounts and delete everything. This will help later if an account has any sort of two-factor authentication or other limits.

Be sure to back up the Mac, and remove any data you want to keep. There is no coming back from what will happen next.

I recommend taking it to the Apple Store at this point, if there is one that is practical for you to get to, and they will help. You'll likely need to transport or pack the iMac for a buyer anyway, so hopefully moving it is not too bad for you.

If that is not possible, you can reboot your Mac and hold down Control + R to enter a "recovery mode," and use Disk Utility to erase your drive. Follow the menu on the screen. If you have not encrypted your drive (and you are not 100% sure that you encrypted your drive) with FaultVault, you'll want to choose the option for a "secure erase." It never hurts to do that, but it could be an overnight process (or more), so please be patient.

See https://www.mactip.net/how-to-securely-wipe-your-mac-hard-drive-before-selling-it/ for an example (Catalina and your iMac will look a little different).

Best of luck!

Doing so will cause the new owner and you missery. Sell the computer with the original version of Mac OS on it and follow the instructions you have been provided above.

You are not supposed to sell a 2014 iMac with Catalina since it didn't come with Catalina.

See: What to do before you sell, give away, or trade in your Mac - Apple Support

Your welcome.