More importantly, if they ever apply for a real estate license, they may be denied due to previous unlawful brokering. 

Age is not a protected group.1.You cannot refuse to rent to someone for the following reasons:a.Raceb.Colorc.Sex (gender)d.Religione.National originf.Disabilityg.Source of income (public assistance)h.Familial status (children under 18 with parent, pregnancy)i.Sex orientationj.Gender identity.2.You can refuse to rent to someone for any other reason or no reason at all.3.If an applicant does not have any of the protected qualities, then they have no basis for claiming unlawful discrimination. 4.The areas of risk are when you are dealing with someone that has one of these qualities.

Going further, if anything taints the legality of the proposed agreement, unlawful dealing without a license, not having the proper business license or not being in good standing, your contract may not be valid.Now, through in the fact that you can only legally sell or lease something that you hold title rights to convey those rights of ownership or leasing.

That then becomes an unlawful foreclosure in attempting to secure collateral, that is a civil matter and can be criminal under state laws.

There are laws regarding each and by intentionally and unlawfully encumbering title, you just might become a Defendant.  

My wife and I went to the courthouse to observe unlawful detainer (aka eviction) cases. 

I know how to get lists from title reps, Melissa Data, List Source, etc but none of these provide things like homes with code violations, Unlawful Detainer Actions (evictions), or deceased owners.

You could be criminally liable if there is a fire and building is used as unlawful 4 unit rental (similar to Ghost Ship fire). 

Also, could the landlord ADD the additional violations - or would they be required to re-file the eviction if they lose the unlawful detainer 1st cause hearing? 

Some states would call it a unlawful holdover.