We bought a used apple laptop?

We have loaded a new system program on it we want to install it but now you need a password to do it. But we do not know the password and also from whom we bought it. What can you do now?

Knock into the bin or sell in individual parts.

The previous owner has set up a firmware password. This means that you can only boot the Mac from a single volume, as an anti-theft protection and therefore no unauthorized person can read and delete data. The only thing you can do is go to Apple with the original invoice. If you don't have it, you can practically throw the Mac away.