Firstly, the concept of 'selling your soul' is pretty dubious, but it is founded in some myth and folklore. It's just not as dramatic as you're making it out to be.
Secondly, the entire point of making a hypothetical pact with the devil, exchangi ng your souls for a reward, is the reward in the case of the mortal, and that persons soul in the case of the devil/demon/spirit/entity.
For one, I doubt such an entity would freely give up a soul without something of greater worth. And someone looking to give their soul up desperately would likely not have a worthy soul.
I for one believe that if such events do take place, entities don't look for any normal stupid mortal to knab the soul from. They would want specific souls for specific reasons.
Whether that be based on the 'worth' of your soul, or some truly alien reasoning. There's a reason any ole person can call out to a demon and offer their soul, yet never hear a reply, yet a rare few men of great talent and ambition do rise to fortune and fame through such methods, at least in folk tales.
I doubt someone looking to simply sell their soul in exchange for anothers would appeal to such beings.
Given, this is all hypothetical and many people believe such things as recent folk superstition, fueled by fictional ideas like horror or the show supernatural (which immediately comes to mind with such requests)...