Rent on a 4 bedroom place may be $5000 monthly ($12,000 in Manhattan); rent it by the bedroom for $1500 per bedroom with access to all common areas.  

How much "fat" do you believe still exists within that? 

Flipping properties, commonly called “wholesaling” can be done ethically, legally and successfully.  

But always complaints about guests (residents) leaving common space a mess, some resident gets mad they have to clean up...   

Make sure everyone knows a cleaning schedule, expectation for common areas, how are you splitting utilities, guest policy, etc. 

But usually for a tenant occupied it's like $90-120 per monthI've noticed a lot of multi-family buildings are pre-WWII housing inventory - are there restrictions or issues common with that (historic districts, crazy high utility costs, etc)?

Lease provisions in one place that are common and acceptable may not be allowed elsewhere.Overall its worked out for us but there's definitely some potential pitfalls.

Their most common threat is a security deposit dispute.

To gain investor confidence, ensure the investment property numbers are real, use the most common contract forms in your state (versus a 2 or 3-page) internet/guru contract form) with appropriate addenda, or an attorney-drafted form and attorney-drafted assignment agreement.

It's common practice in Europe, Canada, Australia but almost unheard of here in the US.