I always wonder with this type of seller, if the repair is so cheap and easy, why didn't you do it before you put the boat on the market?
Let's face, you will be dealing with fittings and hoses that have carried fecal matter. I am not sure how anyone considers that to be no problem unless he or she is missing a nose.
Yeah, I agree, why wouldn't he just fix it? Perhaps he is not handy, or just doesn't have the time, and is willing to discount the boat accordingly.
As for the job itself, sure it is smelly, but that shouldn't be a problem unless you are a serious WUSS!
Every DIY boat owner has to deal with septic system problems sooner or later, and if you don't want to, then there are plenty of people who would be happy to tolerate the smell in order to part you from your hard earned cash! Just use your common sense, wear rubber gloves, wash your hands often and clean up the area with bleach afterwards. On a boat that old you might also consider replacing all of the hoses as well. Boats with old septic hoses develop a certain aroma that permeates everything and doesn't go away. Ripping those old hoses out and replacing them with new doesn't cost very much, and makes a HUGE difference to the air quality in the boat. (At least it did with mine!)