I assumed she passed the screening, but if it hasn't been done, definitely check the credit.  

if the buyers who are calling to see the inside are unknown and the property is $$$, what's best way to screen them, ask for proof of funds or letter from their bank before showing?

Trans union and experian offer low cost screening but with any report you must have the tenants permission .

Clearly this in depth of a report takes time and expertise which is why it is an excellent idea to consult a consumer reporting agency (CRA) that specializes in background screening. 

For that, he'd need a rock star property manager to advertise, show the unit, screen tenants, manage tenants and future repairs.

(You still have to screen them!) 

May I suggest though, if nothing else, at LEAST turn the tenant over to collections and/or report the debt on their credit report so future landlords can be warned (at least those who bother to do proper tenant screening). 

I actually have two long standing violations: my firewood rack is visible from the street and I have an unpermitted covered, screened deck (something else the ex burned up our HELOC on).

I can screen shot the calculation also.

As always tenant screening is important.