Recovery boot old iMac?

Try booting into Recovery Mode using Command + Option + R instead which is supposed to allow you to access the installer without associating the macOS install with your AppleID. Otherwise if you authenticate with your AppleID, then the resulting OS may be tied to your AppleID and cause problems for the new owner. If macOS 10.12.6+ has been previously installed on this Mac, then it will be installing macOS 10.13 High Sierra with this key command.

You can also create a bootable macOS USB installer if you have access to another Qualifying Mac from 2008 to 2018 by using the instructions in this Apple article:

How to create a bootable installer for macOS - Apple Support

A macOS 10.11 installer requires a Mac from 2008 to 2015

A macOS 10.13 installer requires a Mac from Late-2009 to 2018

Hi there, Mark,

Thanks for contacting Apple Support Communities!

We understand that you're preparing your iMac to sell but you're unable to sign in with your Apple ID which uses two-factor authentication.

If you plan on selling your iMac, you won't need to sign into the services requiring an Apple ID. You can learn more about how to prepare your iMac to sell here: What to do before you sell, give away, or trade in your Mac - Apple Support

If you decide you'd still like to sign into the App Store on your Mac, we'd suggest that you contact Apple directly for help with your Apple ID account. They're best suited to assist you. You can find their contact details here: Contact Apple for help with Apple ID account security - Apple Support

Take care!