What documents are filed with the state to prevent the original owner from fraud “selling” the property to another person?

Effectively Screening tenants, following fair housing requirements of course, is THE “key” to prevent problems, as is proactive management and direct communication.

Or is this unnecessary? 

I think you are causing unnecessary concern.

What else is put in place to prevent an investor or lender of any sort from coming after your personal property if you've taken $100k for the deal?

Why get caught up in these unnecessary debates over internal costs, fees, commissions, etc?

As you go through your deal, jot notes in places where you felt rushed, stressed, wasn’t fully capable or didn’t understand a certain process to hopefully prevent snap decision and this situation from happening again.

Apparently I didn't have enough preventative measures against my own stupid feet, though.

What documents are filed with the state to prevent the original owner from fraud “selling” the property to another person?

You are still at fault for not requesting a copy of the document at that time.All I'm saying, all this was preventable if you were to do your part of management.