If there’s an I Ching hexagram and changing line that represents “selling your soul”, it would be Hexagram 27 – Nourishing with changing line 1:
A tortise is an animal that’s able to survive without eating and drinking for a long period of time. In ancient China, the tortoise was regarded as a spiritual animal because of its longevity. The Yao Text, “Putting aside your spiritual tortoise, starting at me with mouth drooling,” denotes that one at this place puts his spirituality aside and watches the person high above eating. A drooling mouth suggests greed. He ought to rely on himself in seeking nourishment for his physical body as well as his spiritual nature. Yet he puts aside his own resources and seeks profit from other people’s toil. This behaviour ends in misfortune. [Huang – the Complete I Ching]
The diviniation of Jou is hilarious:
You are happy with the good and simple way you live, and then one day your pretentious girlfriend drops you for a rich fellow. You think about how lousy it is to be poor, instead of how lucky you are that your simplicity drives bad people away [Jou, The Tao of I Ching]
As for me, I simply describe hexagram 27-1 as selling your soul and losing yourself in the process which results in you falling away as represented by the transformed gua 23.