I am unable to set up a 'used' iMac purchased from a 3rd party

You can not use your Apple ID it the previous owner did not properly prepare it for resale.

see > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

What you need to do is have the original owner properly release, wipe and reinstall or…. you need to return it to the seller for a refund.

If you didn't get this "hello" screen the first time you turned on the iMac it was not prepared for sale properly:

If you paid for it via credit card contact your CC provider and get the charges reversed.

Then you have a paperweight . Apparently you did no do much research about buying a used Mac. Consider this a lesson, hopefully not a very expensive one!

moving forward, always buy from a reputable source if you are buying used!!!!!!!! For example Macsales.com fits that description or buying an Apple Refurbished Mac from Apple directly. Never ever use any of the following:

EBay

MarketPlace

Craigslist

your local computer shop

even your best friend

If it's a problem with Apple ID, then, as others say, you can't change that. Except you can, by wiping the hard drive by booting onto a USB stick with a bootable OS installer, reformatting the HD, and reinstalling a new OS.

Linamercer24 wrote:

I tried that, and she blocked me. 😂

Let me guess. Facebook Marketplace? I have heard allegations that 2/3 of FB Marketplace transactions are scams.

Even though it was written primarily for iOS devices The All Too Common SAD Reality of Buying a Used iPad/iPhone - Apple Community is equally applicable to Macs.

Try using the erase and reinstall directions in the "What to do......" article that I posted above.

If that doesn't help, take it to your local Apple Store or AASP with the sales receipt and see if they can help.

+1 den.thed is correct!

Linamercer24 wrote:

I tried that, and she blocked me. 😂

You got scammed.

Recycle it and take this as a life lesson.