How do I prepare my iMac computer for sale?

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

First, reboot the computer while holding down Command-R. you can let go after the apple logo pops up. this will start recovery mode.

Choose disk utility from the list of apps and click continue.

Find your disk on the side bar (unless you’ve renamed it, it should be Macintosh HD or Macintosh HDD). Click the Erase button after selecting the disk and choose MacOs Extended (Journaled) or APFS, whichever is first recommended. After the erase completes, Press Command-Q to quit Disk Utility.

From the recovery mode main menu, choose reinstall macos and follow the instructions from there.

Note that these are instructions for intel macs. if they don’t work you may have a newer type of mac.

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

Is there an echo in here?

There were no replies showing just before I posted.:(

BDAqua wrote:

There were no replies showing just before I posted.:(

There's been a smattering of that around the communities all day.

Lol same.