The Mysteries of The Soul

What is a soul, actually?
Is it an electrical charge that runs through our body? Is it a swirling energy matter that spins in our body and gives us constant headaches?
Did you know that before we found out how the respiratory system of the body works, our ancestors used to think that when we breathed, we were actually sucking in souls from our surrounding those souls travel through our bodies to keep us alive.
So now, every time we talk about the devil and our relationship to him, we would always say "He/She sold his/her soul to the devil for money and fame." or something like that.
What is a soul? How do you sell it? And why in his own name is the devil so obsessed about our souls?
I mean, what does he do with it after he "buys" it from us? Eat it? If he does then does that mean our souls are like something tasty, like a marshmallow maybe? Do all of us taste the same?
Does he use it to make a bed? Is it something soft and fluffy?
Is it some kind of soap or detergent that he uses to clean his lair a.k.a. Hell? Does it smell fragrant? Do all souls consist of the same chemicals because if they don't then there might be some kind of weird reaction that causes an explosion to happen, you know?
Please don't tell me he takes the soul and puts it in hell because the way I see it, there are enough crooks out there to become his slave when they die so he doesn't have to worry about running out of maids. And why, again, in his own name would he want to find more job for himself and overpopulate Hell? Because theoratically, if Hell exist then it would be a space in a different dimension and that dimension would definitely have a limited area so unless he plans on making a sardine box, I think it's a really bad idea to have so many souls down there.
Plus if the bad souls can become his slave, then that means souls are something that can do work which would also mean it is a physical entity. Oh please don't tell me they aren't physically there and they use their super mind power to get the job done or something.
Why does the devil want to buy souls so desperately?!?!
Oh, and another thing. How the hell do you buy it? Do they use barter system? Like the devil promises to give you instant fame and in exchange you give him your soul? Or maybe they go by cash and the currency is "ghost money", you know, the kind those Chinese people always burn. If we're talking in that currency then I don't know how exactly will the devil buy our souls because I believe those things are really really cheap so our souls are actually millionaires or something.
After careful thinking and scientifical analyzing, I would stick to the barter system.
But that still doesn't explain why the devil has this fetish for souls. My brother says he probably likes eating souls, but I think in truth, he's just buying old souls, reconfiguring them and putting them into new bodies to make sure our world never run out of crooks and dumb asses.

PS: When I refer to the devil as "he", I don't exactly meen "he" as a male because for all we know "he" might be asexual or something. I just used "he" cause it's shorter than "she". Damn it, I shold have used "it"!!