It is presumed that experience has an essential element of identity and this is referred to as a soul. People project upon this a quality of changelessness, and thereby imagine themselves to be transphysically immortal. As a result of this tricky thinking, people join religions promising to secure their future experience in a happy resting or rejoicing place.
The Gospel of Satan
In the fiction of the Gospel of Satan a soul is a ghost inhabitting the meatbody of human animals. Once a human being dies, then its animating ghost or shade is forced through the Gate of Death, which is managed by the God of This World, Satan, unless some other agency such as a god snatches it up and carries it away to some other destination.
Selling the Soul
As part of their negotiations with Christian religion, and as a result of Faustian Tales of cautionary character forewarning Christians about the hazards of dealing with foreign spirits or indeed the ostensible Cosmic Foe of their religion, it is supposed that Satan and other demons are obsessively seeking to collect and control souls for their own purpose, and will therefore make arrangements to purchase this ownership in exchange for what they control.
As part of pact-making and soul-selling activities in Satanism, this notion of a soul is understood as a metaphor for the entirety of one's being and the sum of one's energies. As an aspect and effect of complex nervous systems in mammals, it may be put into the service of another for purpose of exchange of resources or other services in trade.