You can record a memo of your contract, creating a "cloud" that may prevent him from selling.  

Like, if you are a debt collector talking with them about a debt and they are in bankruptcy, then you would be prohibited from speaking with them on any debt related topic.Also, if you are subject to any local or state rules regarding foreclosure prevention companies soliciting borrowers, you might want to read up.

However, lenders have brought to light that state laws prevent them from charging me what they would need to charge me in order to finance this as an investment mortgage.Purchasing it with a conventional mortgage as my primary residence would work (I don't currently own any other property), it is not what I am trying to do here.

Kevin,I've looked for multifamily properties on the MLS on and off for two years while we saved up our initial investment capital, but I've never seen one posted that was in "beginner's shape", so to speak; always large issues with the properties that would either require more starting capital to repair than we have or will be able to leverage, or otherwise have issues that prevent conventional financing.  

Is there some gov regulation preventing it?

My thought about having separate lease agreements (Bedroom A, plus use of common areas, and Bedroom B, plus use of common areas) was to prevent my son from being liable for his housemate's rent.  

In came legislation and HOAs to issue fines and beat you senseless with rules to prevent any real profits.

@Wayne Brooks I'm likely going to have to go that route on closing day if neither have been done, but I'm just trying to find way to prevent that. 

Is there anything that I should be aware or take precautions to prevent these things from happening.

There is no way you can legally prevent investors from moving in on the list you provide them.