Resetting ID and Password on used iMac running OS 12.5

Again, if you can't contact the original owner, the only other choice you still may have it to take it to either an Apple Store or Apple Authorized Service Provider, along with proof of purchase, for them to assist you with this.

Alternately, you can take it back to where you bought it and request a full refund.

If you bought a used Mac, please do the following otherwise your computer will remain a mess as long as you own it!!!!

Please follow ALL the steps in What to do before you sell, give away, or trade in your Mac

An iMac18,1 would be a 21.5" 2017 Intel CPU iMac model.

The following steps would be used to reset the Administrator's account password:

1 - Restart computer and immediately hold Command + r keys

2 - Top Tool Bar >> Utilities >> Terminal

3 - Type resetpassword as one word and no spaces

4 - Press Return key

5 - Follow the prompts and remember the New Password

-- credit: P. Phillips (ASC)

Ref: https://support.apple.com/en-us/HT212190

Just to be sure we understand. Your purchased a used (new to you) Mac, and it still has the original owners user login credentials ... correct? Can you still contact that owner?

Is this an Intel or Silicon CPU model?

You will not be able to keep any third party apps as they do not belong to you and their licenses likely prohibit transferring them. If these are Apple apps and are paid for apps the same thing applies.

Again, if you were sold the computer with the promise of any third party apps, then return the computer and buy a used Mac from a reputable source such as Macsales.com or buy a genuine Apple refurbished Mac from Apple. Trying to do what you are attempting to do will fail (seen it hundreds of times) so please just do as instructed or you will continue to have a LOT of problems that will never get straightened out!

You won't be able to get rid of the previous owners info until you execute the instructions I have already provided. This is what I was referring to, you also will never be able to get rid of their AppleID. You bought a mess, either do what you have been advised or return the thing for a refund.