Could I threaten him with hiring a collection agency who can report to credit bureau ?  

Yes - you are required to report your rental income on your tax return.

The tenant has been living in the unit for 15 years and the inspection report did not say anything about mold.

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I've entered into partnerships where I'm the managing partner and I need help to make sure I'm reporting the financials correctly.

If  you find fraud, that is another issue and can actually be reported to police. 

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Zillow states the following on their annual report, "If Real Estate, Rental and Mortgage Professionals, Home Builders or Other Advertisers Reduce or End Their Advertising Spending With Us or if We Are Unable to Effectively Manage Advertising Inventory or Pricing, Our Business Would Be Harmed."

You can choose the "carrier routes", which are a partial zip code, based on the renter/homeowner % reported in the last census.

Not the best news, but I'll still give it a go since the current rate is so much better than what I'd get otherwise.