'do not make any impolite or rude comments' don't tell me what to do, you're not my father! [i'm joking... and i'm pretty sure you're not my father...]
well ancient mythology are religious, but they tell stories and give life lessons. in this respect, the belief one can sell their soul to an evil being brings misfortune is a way to warn people that if something seems too good to be true, it is. i personally do not believe you can sell your soul to anyone for anything. anyone who tries usually does not grasp the severity of this exchange. sure a billion dollars or your celebrity crush in exchange for something you theoretically aren't using might tempt many, but a lifetime is so short, so anyone who tries selling their soul, imo, hasn't really thought it through. even those with noble intent [like my life for my loved ones] might want to think about it further.
the soul to me is the spark of the divine that wishes to experience3 all things, while i am me in this lifetime, i do not own the soul, when i die it will be reborn as something else. [energy just changes forms] i do like the notion you are reborn around loved ones from past lives, but that's just a personal comfort and not a universal belief. i don't believe it can be lost, but if it could be lost, it's not really worth loosing because, again, it is a piece of divine energy, so it is [to me at least] energy from the God and Goddess. you loose it, it might become something else [essentially it won't die] but you will feel different and your life will become different. i'm not saying you'll become a zombie, or that those who lost faith lost their soul, but i feel your life will become muted [if that makes any sense]