I was hoping that we could see how the two spend their last three days and how the community would react to both their simultaneous deaths. Well, at least I don't have to shed another tear.
@crynelleciaz Pretty good theory. I thought it was weird that a father would talk about shortening his life in front of his daughter but then the girl mentioned "mister" and not "dad".
However, one plot hole to your hypothesis is that if he had sold his lifespan, he wouldn't guess zero. Why? Let's say that he had a value of 'x' to his lifespan. Logically, if 'x' were to be high enough, he won't be selling old books. Intuitively, if 'x' were to be low enough, anyone would think that value is the lowest it could be. If 'x' really is zero, that means he sold all of his life, except for one year, for zero dollars; his observer is this statement's evidence, according to your guess.
It's more likely that as
@datamaru stated, the mangaka just wants to retell the essence of this story in a shorter format with a different perspective.
But at the end of the day, those two are mere background characters in a chapter that is just an extra; they may have a deep personality and a definite backstory but there isn't a point to inspect these people further since this is truly the end of the story.