MacBook Pro locked using Find My Mac
Unfortunately, it seems that you purchased a system that had Find My Mac enabled. This leaves you at the mercy of the iCloud account owner to not lock the system. I doubt there is anything Apple can/will do to help you unlock it. They may be able to help locate the account owner.

Advice for next time: Always check System Preferences to ensure that Find My Mac is disabled before handing over any money. To be completely safe, boot to Recovery Mode by pressing Cmd+r at the startup chime. Enter Disk Utility and format the Macintosh HD. Following that, use internet recovery to reinstall macOS.

That might seem like a lot of work, but it ensures that there is no malware on the system left behind by earlier users of the system, whether intentionally or otherwise.