Deleting old data prior to sale

This is what you have to do: What to do before you sell, give away, or trade in your Mac - Apple Support

If this is an Intel Mac, then I personally like to select the whole physical drive to erase so that a new partition table is also recreated, but I believe you have done what Apple's instructions mention. Within Disk Utility you may need to click "View" and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. Erase the physical drive as GUID partition and MacOS Extended (Journaled) for macOS 10.11 to 10.13, or as GUID partition and APFS (top option) for macOS 10.14+.

Does your Mac use a hard drive or an SSD? If it uses a hard drive did you have Filevault enabled? If not, then you need to write zeroes to the whole hard drive in order to overwrite all your personal data, but with recent versions of macOS it is not easy to write zeroes to the whole drive as it requires using the command line. If the Mac uses an SSD, then you are fine due to how SSDs work.

Here is an Apple article on things to do when selling a Mac, but the last step for performing the clean install assumes the Mac uses an SSD or that Filevault is enabled if using a hard drive which is why I provided the extra information above.

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

Hi MDJ1984,

You should erase your Mac again. If "Other Volumes" appears, something is likely wrong.

To erase your 14" MacBook Pro, follow these steps:

1. Open System Preferences.
2. At the top of the screen, select System Preferences -> Erase All Content and Settings.
3. Provide your admin password when prompted, then follow the onscreen instructions to completely erase your Mac.