wiping my iMac to sell
I need to sell my iMac and want to make sure I've saved all its files and then reset to factory
Question marked as
See the following Apple Support article for information on erasing your iMac before selling: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
To back it up, see: Back up your Mac with Time Machine - Apple Support
Since your post references iCloud (top left of this screen), also see the following:
Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.
It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.
Also see the Apple article: How to back up your Mac - Official Apple Support
And more information on syncing in:
Set up and use iCloud Photos - Apple Support
and
Access and manage your files in iCloud Drive - Apple Support
and
Add your Desktop and Documents files to iCloud Drive - Apple Support