No way he could have grown his rental business that quick or developed scale, by buying his own properties.  

This listing is off the MLS.

I would add that you should definitely checkout some local real estate meetups as that's the best way to quickly build a buyer's list. 

I liked real estate so I got licensed, worked on a bunch of different RE strategies, started developing and managed to get myself stretched thin very quickly again, I basically made the pendulum swing way too far and had to consolidate again to find my path.

We have a tons of lots around here like the one you described, many of which can be bought for cheap and are not listed because there are just barely any buyers for such.

I would also take a look at other listings in the area.

What we really need is a single internet "bad tenant" list to warn each other.

And lastly, I search FunishedFinder listings nearby, find comparable properties, and track the listed rent for a few months (does it increase, decrease?).

Try listing it locally or selling to tenant?  

I had grown very close to him and had developed a great deal of trust for this person.