I'm reading Shannon Wheeler's Too Much Coffee Man: Cutie Island and early in the book, TMCM sells his soul in order to have his book be successful.
That's not the epiphany. It's actually sort of an obvious observation (we've all talked about people "selling their souls" to the corporate world), though Wheeler makes it in a very clever and profound way. He caught me off-guard by first presenting a story in which TMCM sells his soul to a literal demon, then waits until the last minute to explain what that symbolizes (or that it actually symbolizes anything).
Even though I mentioned the corporate world above, this isn't to equate all corporate jobs with demonic servitude. I've had corporate jobs (and have one now) that have brought me a lot of satisfaction and joy. Notice that TMCM never says what kind of job he has that he hates, because that's not important. It's the fact that he hates it and has sold out to it in the hope of getting what he thinks he wants. He's betrayed himself and so, sold his soul.
The epiphany that this all led to is the idea that demons are actually representations of our own self-destructive tendencies. In popular culture, demons are the manifestation of ultimate evil and evil is best defined as ultimate selfishness. Selfishness, when allowed enough freedom, is extremely harmful, not only to other people, but also to the person who's being selfish. It destroys relationships and fails to compensate with anything truly meaningful.
Seen that way, selling one's soul to the Devil is simply a metaphor for selling out to your own selfishness. "This is what I want and I'm willing to do anything to get it no matter whom it hurts, including me." It's a bad bargain that never pays off.