There are distinctions to be made in the Neath when it comes to the nature of Death, your Soul and your conscious Self. The Neath is strange, and it is possibly the nature of death that makes selling one’s soul something that won’t outright kill you.
However, a soul is still a soul and it’s importance to someone is based on personal belief. Even if a person is not of the opinion that the Heaven above the Surface is all that it is cracked up to be, perhaps whatever the denizens of Hell have in store for it is much worse (presumably eating it). There is no moral or sensible reason to sell your soul, it is what makes you human until the very end when the Boatman takes you for one last journey.
The ‘you’ that exists outside your body and without your soul is a mystery - but it may be connected to the reason why most cannot go to the surface after having died once before in the Neath.
Something down here is keeping us here…