iMac in recovery mode, can’t install iOS no matter what I try!

My wife and I purchased an desktop iMac a1311 from Facebook market place. It was perfectly functional but the seller hadn’t removed any of their personal info before selling it so my wife erased it (I believe it was a factory reset that she did). The computer now only boots in recovery mode, and we are unable to reinstall the operating system because it’s requiring the password of the person that sold it to us. We’ve tried contacting them for either a password or even proof of purchase to bring it to Apple, but they will not reply.

So currently we are trying to reinstall the iOS from a bootable thumb drive. I succeeded in making the drive from another computer, but that’s only lead me to more hiccups. For one, I can’t get the computer to start up in the boot menu. No matter what keys I hold down at it opens in recovery mode.

I thought I figured out how to work with this when I went into the operating system installer and saw my thumb drive as an option, but when I click on it, it says the disk is locked. So I’ve been trying to figure out how to unlock it, including the verify and repair, but when I clock repair it says it cannot be done.

So help with any of this would be greatly appreciated. Either getting to the boot menu, unlocking the disk, or even better if there is some other easy solution to getting around the issue with the password to get the iOS up and running!