Our realtor team is acting as the mediator between us and he believes it's condensation from the ductwork, which is in-between floors.

The heat will create condensation on the wall and the dirt/dust sticks to it.

They didnt want to pay to replace the air conditioner so they had a condenser installed which the contractor said wouldn’t work but we did it anyway.

buying the wrong properties to start with in the wrong areas.. low end that look great on paper.. but the tenants are sporadic at best move every year almost always.. and when the house does go vacant they get vandalized  condensers stolen that's what cost in some areas.. to fix that I think you just need to buy at the top end of the rental price ranges so you can attract that long term stable tenant..  

I've gone through their military house hacking course and its very informational and condensed.

They can also go in closets and they're not vented, they condense the moisture and drain it out. 

I've looked in the condensation covered windows and noticed mold covers about two feet up on the walls and the ceiling above the stairs has fallen down. 

Getting air flow is key to remove any condensation.  

So this is a long story... but I wanted to ask people's experience, if anyone has some.The VERY condensed version is that I am potentially going to purchase a house/property from my uncle. 

@Yuvaraj Vimawala,We had a similar situation, in mid-May, when it was like 90+ the AC unit went out... except ours was brand new, and apparently it was just a bad condenser coil... and the part had to be special ordered, and then the HVAC company (which charged us $140 to diagnose it, and everyone else was charging at least $100 to even look at it) said they couldn't fix it until the very end of June.