First, your iMac had a Fusion Drive setup where it used a small fast SSD combined with a large slow hard drive. During your clean install you temporarily "broke" the Fusion Drive setup and are only using one of the two drives.
Second, it appears this iMac was not properly prepared for sale since it is still being managed by a previous owner. You should return the iMac for a full refund since you don't know if this was just an oversight by a previous owner, or whether this iMac is stolen. This is one of the dangers of purchasing a used Mac. You could try contacting the company in Seattle to see if they will remove the iMac from their MDM assuming the Mac was not stolen.
FYI, if you need to restore the iMac with an OS before returning it, then you can use the instructions in this Apple article to reset the Fusion Drive and reinstall macOS again.
How to fix a split Fusion Drive - Apple Support
I'm glad you did the clean install to make sure there are no nasty surprises left behind within macOS, as it also allowed you to discover that the Mac is still being managed and was not properly prepared for resale.