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Have the seller do this: What to do before you sell or give away your Mac - Apple Support

The previous owner did not properly erase and prepare the MacBook Pro for selling or giving away. The article that BobTheFisherman linked to outlines the steps that the owner should take before selling the MacBook. Here's a condensed version of what you'll need to do before using the MacBook Pro normally. (You'll want an excellent Internet connection for this).

1. Boot Internet Recovery Mode by holding down Option-Command-R. (This will take a while.) If prompted, select a Wi-Fi network and continue. If you are stopped by a padlock instead, return the MacBook Pro to the previous owner and ask that he/she remove the firmware password.

2. When the Utilities menu appears, choose Disk Utility. Then, erase the entire hard drive by selecting the top level of the hard drive, clicking Erase and choosing these options:

Name: Whatever you want, but I recommend Macintosh HD

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Table

3. Quit Disk Utility, then choose Install macOS from the Utilities menu.

4. Choose Macintosh HD (or whatever you named your hard drive) as the target, then let the installer download a fresh copy of macOS High Sierra and install it on your Mac.

5. When the installer is finished, your Mac should restart to a setup assistant. It will guide you through the steps of setting up the MacBook Pro as your own, and you will be prompted to create the first account on the MacBook, which will automatically have administrator privileges.

Hope this helps!