Mythology is replete with examples of people selling their soul to the devil. Except... how does that actually work, mechanically?
Let's say you sign a contract agreeing that the devil may take your soul. How does it get transferred to him? If he has the power to take it himself, he wouldn't need a contract. So maybe it's something you do, like how when you sell someone your car you have to physically hand them they keys. I don't know about you, but I wouldn't have the first clue how to go about handing someone my soul.
If it's not the devil that removes it, and it's not the contractor, that means there must be some third party. An external agency, that manages and administers the soul contracts, ensuring that the deal is fulfilled, and transferring the soul to the recipient. A neutral party, that can be trusted not to cheat, and is not interested in taking the soul for themselves. Possibly an individual, possibly an agency, but there's got to be someone.
So, who does this? And what do they get out of it?
Edit: Apparently I need to be more specific with what I am asking
- Assume that it is possible for a human to sell their soul to a supernatural being.
- Supernatural beings interested in acquiring souls are not able to take them by force, and humans cannot transfer souls without assistance.
- Therefore some other supernatural being (or organization of such) mediates soul-transfers, and enforces such contracts (i.e. rules whether the terms are met and the soul transfer goes ahead).
- What is the identity of this being(s)?
- Why do they do this? How do they benefit from assisting in the transfer of souls?
So, although additional details are certainly welcome, a basic answer might look like "soul exchanges are mediated by [NAME], who do it because [REASON]".
The best answer is the one that requires the least changes from standard mythology. That is, as much as possible using existing mythological beings, with an explanation that is similar to their myths/domain, rather than postulating an entire new cast or ascribing out of character motivations.
Apologies to whoever this was not clear to.