The Zohar, Chapter “Chayei Sarah (The Life of Sarah),” Item 155: …Hence, the righteous purify their bodies, which come from the force of the left, until they are worthy of receiving the Mochin from the middle line.” This is already the Light of Hassadim, with a little Light of Hochma clothed within it. “And upon the reception of the Mochin, the Guf that despises its authority is like Esau. And an impression of that degradation remains in the body after its passing, too, until it causes it to decay, meaning to completely annul its power of reception until no remnant is left of it….
Esau is my inner decision. It is when I begin to understand that I can no longer work with my desire to receive pleasure the way I previously worked with it, meaning for the sake of receiving egoistically. Moreover, this decision is final and irreversible.
“Selling the birthright” means that I do not use my desire to receive pleasure in its primal form, for the sake of receiving. What I “sell” is not the egoistic desires (Kelim) themselves, but the system of their use. With this my previous attitude towards life disappears.