'Selling Your Soul'

There is no way you can 'sell your soul', as it is an eternal incorruptible fractal of GodSource. However - as this fractal has 'free will' - it can enter a contract with an entity - to relinquish this freedom and serve the entity - sort of 'leasing the soul'. This contract can be revoked at ANY TIME asserting one's sovereignty as part of GodSource. 

The catch is, that you will enter such a contract only in a delusional state, with no awareness of one's true nature. In this state you can BELIEVE that you are now eternally bound to that contract and all access to your divine nature is severed forever. As long as this belief persists, you are indeed bound. Also your contract partner will amplify that conviction and reduce your mental clarity eg by the use of drugs, instigating you to crimes, etc. apart from the punishment that is usually part of the contract for breaching it.

So this bondage will last until one comes to a moment of clarity - from remorse, total exhaustion or another situation, which 'breaks the spell' - to sincerely reverse the original decision. One may have to endure the karmic consequences of the actions during the time of servitude, but  once one reasserts true nature, there is nothing any entity can do, to prevent it, as every being comes from GodSource and is bound by it's laws.