When a person sells his/her soul, How does the buyer determine the used sale value of the soul?

Is it the unused sin capacity of the soul? The desperation of the seller? Is the mortal social status of the soul carrier taken into account (pope's soul worth more than a gansta thug). Or perhaps the purity of the soul itself is the only criteria.

But if the purity is the major determining factor, is a pure soul of a young child as valuable as a pure soul of an 100 year old person?

My wife tells me I think too much, but some times an inquisitive mind need to wonder, or is it wander?

Remember this: Everytime I blink, you disappear.