I asked several times about verifying due diligence etc and it was never highlighted as something that has brought up any major issues for anyone and they say they have professional and trained staff LOL.

However, they are self-employed.What is the best way to verify income for someone that is self-employed?

Obviously, I am not an attorney, verify all of this information with an attorney to help with your specific situation.

Obtain proof of verifiable income, which you then verified?

I would do the following, at a minimum:-request that he complete your rental application (hopefully you have a thorough application),-request identification documents (government-issued),-require that he provide you with proof of his visa status and the authorized length of stay,-verify his employment, income, and financial stability, and-contact his previous landlords for references.If he cannot satisfy these application requirements, do not approve.

Make sure there is adequate parking for multiple units and verify local zoning rules.

We verified with the police that the unwanted guest was indeed still living in the rental with the main tenant, and that the unwanted guest had had a case of an overdose just a month ago in the rental.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

If not, its iffy, have seen it go both ways, but the long time that it has been verifiable (1972) speakes strongly in your favor.

You’d have to verify their claims, as you are - wisely - saying you would.Good luck.