How do I add my own Apple ID as main one and remove the previous owner on iMac?

If the seller is willing to help your husband make the Mac usable, make backups of any of your own data – and then have the seller prepare the Mac as outlined in:

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

If this results in the Mac being reset to an older version of macOS –e.g., the one it originally shipped with – you may then want to upgrade macOS as far as that particular machine supports afterwards.

How to download and install macOS - Apple Support

Then reload your data from your backups.

Please have your husband return the computer otherwise it will ALWAYS be associated with the previous owner!!!!!! The computer was not setup correctly for resale and will always be a problem. Please tell him to please buy second hand Macs from a reputable reseller like Macsales.com or from Apples refurbished store. NEVER ever buy buy used Macs from:

• E-Bay
• Nextdoor
• Marketplace
• Craigslist
• Friend
• A friend
• Local PC repair shop
• etc…

Why because almost always there will be some major problem such as what you are now experiencing. Ignore ur as s146’s advice , it is wrong!!!!!

"Ignore ur as s146’s advice , it is wrong!!!!!"

And was likely produced by some skanky AI app, not a real person.