Someone suggested maybe a humidifier or maybe condensation from the bathroom. 

They are coming up around the drain pipes to get to condensation.  

A true "ventless dryer" will not have a vent that needs covered up, since it uses a condenser to get rid of moisture that usually is put out through the vent.

The furnace condensate line needs connected into the floor drain, and a temporary thermostat supplied.

A traditional split system with an indoor air handler and a variable speed fan does an excellent job of removing humidity from living spaces (as evidenced by water in the condensation pan), but I always wonder if mini-splits do the same.  

AS stated 2 thefts are a harbinger of things to come.. cheapest 1000 you ever spent if you change your mind I will bet.. there is a lojack for condenser units.. its one of the first things I look at in the areas we invest.. are the units caged.its usually the copper they are after you need really tough copper recyle laws to combat this like in CA .. in CA copper theft is automatic 5 years in jail...

To replace the furnace and the condenser outside will run you a good few thousand dollars so negotiating that into the price will help for sure.

This actually helped me out recently when the condensate line got clogged in the HVAC closet and the water puddled and set off the water alarm.

The air condenser is in visually poor condition and my assumption is that it will need to be replaced in the coming years; however, it is currently working great.I'm currently leaning towards replacing the fence as it would create the most curb-appeal for potential renters. 

I'm not a huge fan of window units for security reasons & I commonly see window units installed incorrectly so they wont drain the condensation out well.