How to set back to factory settings nfor. Selling
How to set back to factory settings for selling
iMac 27″, macOS 11.2
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Have you read through the "Related Article" linked at the top of the page?
There is also this: What to do before you sell, give away, or trade in your Mac - Apple Support
It doesn't matter whether it is for selling or not selling or doing whatever. That is how you erase the Mac and get it as close to what it was when it came out of the box.