Then stain the walls and floor.

I prefer sealed and/or stained concrete. 

The house still has the old pukey green carpet that was probably in it new 30 years ago...yes stains and all.

Water stains are visible along this wall from the ceiling (two-story room) all the way down to the floor, with corresponding floor damage.The house was built in 1994. 

If not at least you are a stain on their credit they will need to explain when trying to get a loan and you are increasing their rates.  

No fences to have to upkeep, a basic deck on the back of them, all concrete stained floors, wooden cabin looking walls so no Sheetrock or paint to be done.

There will probably be beer stains on the carpets, at least 1 hole in the drywall and various other issues, when they move out. 

He said he has the following issues with it and hasn't taken good care of it the last 15 years he's lived there (also built in 1977):- rotted wood all throughout the roofline- brick wall venier on one side of the house is falling apart- sheet rock in many places needs to be replaced- cabinets have never been painted or stained- carpet is over 20 years old- linoleum floor in the kitchen and hallway is yellow and peeling up- walls haven't been painted in 15 years- septic issues- a couple of leaks in the roof- possible insect / termite issuesSupposedly a lady has offered him $80,000 for it. 

They just do not stand up to normal use.Maybe a stain but not paint.Consider covering the floor.

For example, I have had people say that Kool Aid stains are normal wear and tear.