Stepping back I realized it'd be better to look out of town before jumping into this only because it's local.I sent the seller a form letter backing out politely and let them know I'd be interested as a cash buyer if they ever needed one, but probably at a price that would be offensive currently, and wished them the best on selling their property quickly. 

Both report a good income  well over >3x monthly rent, but both have criminal background history and low credit score (upper 500s).

I don't want to sound offensive here, but have you tried youtube househacking tutorials?

It costs like $30 or $35, and gives you a lot of info about the person, including criminal, credit and eviction history.

I just need for you to understand the seriousness of the situation.Also criminal charges , ugh let's hope it blows over" Because it's a service dog I couldn't get them to sign the pet addendum or anything to protect myself from the actions of this dog.

Go on the offense and watch your experience and ROI grow.You've got this...

From the other point of view - how do I, as the owner of the closet/garage/basement, know your "stuff" isn't dangerous, illegal, filled with roaches, or that you aren't a criminal seeking access into my property?  

I always have tenants pay at lease signing; because it amazes me how often people will submit an application, pay the credit/criminal check, tell me how much they love the apartment and then disappear and not answer calls or texts (they either died, found a different place, or mommy's no longer kicking him/her out of the basement). 

Criminals are quite lazy by nature, and many will look for lower hanging fruit.Check the storage units supply companies online. 

Unless you go on a solid recommendation from someone you trust, you really don’t know if a Private Money lender is real and trustworthy or fake and criminal.