Your best bet is to ensure that there are no material modifications that would in any way harm or even moderately delay the ability to rent it out and qualify as a normal SFR (one lease, one family) or sell on the market to an owner-occupant

My unit sits vacant longer and this poor family will struggle to find a good place to live.

Who wants to bet the daughter has maxed out student loans, they both have iPhones, and make needless purchases? 

1) Rarely paying asking price or even "market price" for a property.2) Understanding how they may be able to reposition a poorly managed property for increased cashflow and value.3) Looking for creative financing, so they don't have to deal with a bank for a purchase loan.4) Understanding ROI, Cash-on-Cash, NOI, etc. and using those numbers to determine HOW MUCH they should offer on a property to meet their taget performance numbers.Funny, appropriate, recent interaction with a plumbing company.

Picking a house with major, costly structural issues or in a location with poor rental demand can hurt both the rental and refinance phases.

Good reads would be Rich Dad Poor Dad by Robert Kiyosaki and How to Invest in Real Estate (the beginners guide) by Brandon Turner.

Also, I like your podcast; it's been really helpful.The need to point out what someone doesn't know and how poorly they have constructed a forum post doesn't help anyone. 

(Property is already in poor shape).

Unfortunately, to my knowledge, I don't believe there's any documentation that a future landlord might find to tip them off to the poor character of these tenants.Moving forward, I'm less dismissive of warning signs in the tenant screening process.

I bet passive investors had been working with coaches to underwrite the deals they've been investing in the past two years.