I'd deny application due to information found on the credit report/background check.

The application was denied on the fact they had no credit.

That seller will be required to give access as you can't deny access to a property you sub-divide, but also;2.

The the real estate professional status is one of those categories that gets audited heavily as many abuse their status.

I would recommend getting a credit card and paying it off monthly if you have the discipline to not abuse it.Foreclosures can be tricky.

This does not sound like the case with this seller but here we have a lot of out of town buyers that get tempted by the cheap looking 4 families and buy them figuring what can go wrong then they get abused by their tenants and their property managers.  

As long as the owner asks for SSNs from all applicants (and why wouldn't they, building systems and process is the way to grow and streamline your business), denying someone tenancy based on the fact that they do not have a SSN is not discrimination.

I currently have an applicant that I'm pretty certain I'm going to have to deny based on a previous landlord's reference.  

(She was the abuser.)

I think you can deny based on occupany quantity. your town may have restrictions as well that you can put on your side.