No stains just the lingering odor that I don’t want to deal with.

There are all kinds of tips on cleaning old oil stains off concrete...soap and power washing works well.

However, on the inspection reports the inspector listed one unit has " a tobacco odor and stains present throughout the unit" and the other has "tobacco odor in unit that seems to be coming from the neighboring unit." 

Note, also, that he common area is also in horrible condition: Matted, stained, frayed carpet, beat up wood, hideous/peeling wallpaper.

It's not uncommon for tenants to try and remove a stain using bleach.

My "non professional" opinion is just staining of 50 year old wood there is no fuzzy mold.  

We ripped out all of the carpet and had the concrete stained/polished and then painted the interior and exterior.

Wait, so did they want to buy the house before but now they don't because of a stain/scrape on the floor from a kitchen repair? 

The koolaid stain may require white vinegar to be poured over the stain for an hour but it can come up toonif it’s not too bad 

Another issue was he wants the deck stained because the boards are dry and cracking on the decking boards and the sides of a screened room.