If you did not trigger the lock/disablement, then it means a previous owner still has control of the laptop. If the previous owner gets the laptop back, then they may be able to unlock the laptop (or have Apple unlock the laptop if the person can provide valid proof of ownership). I know at least one of our organization's laptops had the "Disabled screen" triggered somehow (not by us - I believe it was some fluke) that would never provide us the opportunity to enter an unlock code and supposedly a co-worker performed a PRAM Reset which allowed the laptop to boot normally again. Otherwise, in theory, no one should be able to access the contents of the laptop. However, depending on the exact model of the laptop it may be possible to remove the drive and access the data on it. When someone has physical access to a computer all bets are off since a person then has the means, especially using specialized tools & hardware, to take advantage of any system vulnerabilities.
Your story shows that it is very risky purchasing a pre-owned Mac from third party vendors. Even if a user attempts to perform a clean install of macOS by first erasing the drive, it may not always reveal whether the Mac is still being managed by a previous owner since some of the management notifications may not appear for days or even weeks.
The short answer is your data is probably safe especially if the drive is encrypted, but the encryption is only as good as the password used for the admin user account(s) -- the security is the same as if the laptop was lost or stolen. There are absolutely no guarantees though.
