Contact a debt collection company and have them handle it for you. 

If this correct, why people buying 100 units with multi millions and only collecting 4 or 5% cap?

To the OP  I high suggest you get on Net flix and watch the American Greed Episode on Remington Financial and or Google Ingrid Robinson  Remington financial and read in depth how these pump and dump fake lenders set them selves up to just collect up front fees and never make loans.. 

My offer was rejected so the owner is now collecting less rent, but still has the same expenses.

If they are paying, just keep collecting.

Harder to finance.Less appreciation.Tenants that talk to easy other.Higher maintenance.Higher turnover.And you have 4 rents to collect, and most likely one will be late.Yes, you can get a bit better rent per price than you can with SFH, IF you can collect it.And not sure what a C neighborhood is, but not good enough.BTDT, gave my last one away..... years ago.

*** What do I need to do from a licensing standpoint to collect leasing commissions for vacancies my team leases up?

---> I think I will have some more questions when I get to this part on what to include and what not to include in the lease- Set up rent collection ---> Appreciate recommendation on which tool or site to use for thisGuys, this is my first time being a landlord, so I would really appreciate feedback, recommendations and forgive me for any dumb questions.

@Brian Garlington is spot on with verifying that rent is actually collected.  

There are tons of people who invest there with hundreds of years of collective experience.