I've never heard of actually being able to "sell your soul." In a somewhat related way, I do believe you cannot lose your salvation (i.e., once you are saved, there is nothing you can do to lose your place in heaven, not murder, nothing) and that salvation is always an option, no matter what your past may have been (i.e. murderers on death row making last-minute confessions, etc.)

As far as possession, I have known people who have professed to believe in it, as do I - I was friends with someone in college whose house, I believe, was posssessed (some weird stuff went down in that place). Unfortunately, this would just come off as heresay - I will look for more concrete resources though.

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"Life's a b*tch so I became one" - Rusty in the movie Flawless