locked disk

You should return the laptop for a full refund since the previous owner did not prepare the Mac for sale by following the instructions in this Apple article:

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

If they neglected these steps, then the Mac may even have a firmware lock enabled or it may still be managed by a previous owner. It is easy enough to detect if a firmware lock is enabled, but it is much more difficult to detect whether a Mac is still being managed by a previous owner (and you may not know until days, weeks, or even months later when it is too late to return the computer).

You should always perform a clean install by first erasing the whole physical drive before reinstalling macOS so that you have a factory fresh OS on your Mac even if the previous owner may have done so. You have no idea what nasty surprises the previous owner left behind. If you are booting from a macOS 10.11+ installer, then you will need to use Disk Utility to erase the whole physical drive. With macOS 10.13+ you will need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. With macOS 10.11 to 10.12 you need to erase the physical SSD as GUID partition and MacOS Extended (Journaled), but with macOS 10.13+ you need to erase the whole physical drive as GUID partition and APFS (top option).

Performing a clean install will at least alert you to whether the Mac has a firmware lock enabled. If a firmware lock is enabled, then you will have no way to disable it since you won't have the necessary proof of ownership to have Apple or an AASP to remove the firmware lock. You will need to return the laptop for a full refund.

If you perform a clean install of macOS, then you may receive a notification the Mac is still being managed when booting the fresh install of macOS, but sometimes you may not see any notifications until days, weeks, or even months later when macOS pops up a notification while using macOS (assuming you even leave notifications enabled). Again, you will not be able to do anything about this yourself so it is best to return the Mac if you see this notification since a previous owner could control the Mac and even lock it remotely.

Even if the previous owner provides a password to unlock the Mac, you definitely need to perform a clean install of macOS. Do not use the Mac as is with their software & settings even if you change the password as it will end up causing you problems. Plus it won't alert you to the two potential issues I have already described.

Be careful.

ProphetRay Said:

"locked disk: i cant get my used Mac mini 2014 to install High Sierra anyone know how to fix this?"

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Enter your Drive's Password:

If you are locked, then it seems your drive is encrypted with a password. So, enter the password. Don't know it? Maybe it is the name of a teacher or a pet? Just think of passwords you've used in the past, or are currently using.