Don't know Macbook login password

Recently I bought a used Macbook air but it was locked. The seller was not reached by phone or email. So I tried to factory reset or re-install the OS but all failed due to various reasons. I called Apple care, but they seem to misunderstand my question and tried to reset the password of 'my' Apple ID which is totally irrelevant. They said they cannot reset the seller's Apple password. When I failed three times in a login window, they gave me the serial number (of the Macbook) and the record number. Can I reset the login password using this information? What can I do to use this laptop?

By the way, is the computer log-in password same as the seller's Apple ID password???

Solution 1:

I don't believe a Mac can be locked to the point where you cannot even install a fresh OS. That would require a locking at firmware level, and I don't believe the MacBooks offer this option.

You should be able to restart, with the option (alt) key pressed, so that you get into the boot screen where you can choose to start up from the recovery system, and then re-install OS X from it, all without needing a password.

Oh, I see you also asked here: Resetting password for Macbook There, you mention that you were able to get to the rescue screen with Cmd-R. So, why can't you re-install OS X then? It's offered as an option there, isn't it?