@Diana Mulvihill A lot will depend on your local market and competition.

I am an experienced investor and unexpectely find myself managing a duplex myself and wondering if there is a small landlord (aka inexpensive) simple solution for marketing, background checks and lease creation that markets on zillow? 

You're in a strong rental market, and house hacking a small multifamily in Phoenix is a smart way to build wealth.

While I'm well-versed in the Australian market, the U.S. market is completely new to me.

Quote from @Sebastian Medina: I would search out people in your market that are flipping, and try going to a couple meetups as well.

Are the tenants paying market rents?

If they tenant is $300 below market, that's costing you $3,600 a year.

We do not raise it to market rents, we keep it at lease 10% below to try and reduce turnover.

In the future I would like to run Facebook and instagram ads in a market that guest from.

Depending on the condition you'll command a better price on the MLS vs. off market.