Used mac

It is better and safer to perform a clean install of the OS by just booting into a macOS installer and erase the entire physical drive before installing macOS. You have no idea whether the current file system or macOS are Ok and the previous owner may have left behind some nasty surprises for you. Plus performing a clean install can reveal whether the Mac has a firmware password lock enabled and whether the Mac is still being managed by the previous owner.

You can boot into Internet Recovery Mode using Command + Option + R to access the online macOS installer. Depending on the exact model Mac and the operating systems previously installed you should in theory boot to the most recently supported macOS installer for that Mac. Many times, however, only the the original OS which shipped from the factory will be offered, but at least installing the older OS will allow you to access the later versions of the macOS installers from links within this Apple article which also contains optional instructions for creating a bootable macOS USB installer:

How to create a bootable installer for macOS - Apple Support

If you are installing macOS 10.13+, then within Disk Utility you will first need to click "View" and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. Select the physical drive which should be the top most item which is usually identified by the make & model of the physical drive. Erase the physical drive as GUID partition and MacOS Extended (Journaled).

Contact the seller for admin PW, or See if you can create a new administrator account by restarting the Setup Assistant:

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Be very careful to notice the spaces in those Terminal Commands.

Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

BDAqua, i was having this same problem with upgrading my MacOS (no administrator). I tried your suggestion, but after step 2 above (….fsck -fy), i get the following:

warning: option -f is not implemented. Ignoring

error: container /dev/rdisk2 is mounted with write access

Any suggestions for how to fix this?