Basic Trope: A character sells his/her soul, independence or something extremely important to Satan (or a similarly 'evil' or 'trickster' mythological figure or a mundane powerful and corrupt slimeball) in exchange for something else.
Bob wants to be a millionaire and win the love of Alice, so he sells his soul to Satan to achieve this.
Bob operates a restaurant and makes a deal with a Mafia Don to obtain funds for its expansion and to destroy the competition, in exchange for his allegiance.
Exaggerated: Bob wants to be omnipotent ruler of the world, so he sells his soul and the souls of everyone he knows and loves to Satan.
Downplayed: Bob rents his soul for a for trivial and short-term gains.
Old Mr. Scratch is very powerful and capable of granting any wish — for a price...
Bob is an extremely bad situation and has to resort to this as a form of Deus ex Machina (or, more accurately, a Daemon Ex Machina)
Bob sells his soul to Satan for a wish that he will make at a later date. Happy at getting Bob's soul, Satan goes to Vegas, gets a hangover but is pleased nonetheless. At a later date, Bob makes a wish and Satan realizes that the wish is way bigger than he can handle. Due to the Vegas trip, Satan neither has Bob's soul to refund nor the resources to grant the wish since he lost it all while gambling. Because of legal stuff, Satan has to fix this mess otherwise he'll either be Bob's eternal slave, be fired by the bureaucracy, or both. To fix it, Satan concocts a Zany Scheme which involves lots of cardboard and ironically a Deal withCthulhu.
Zig Zagged: Bob is presented with the chance to sell his soul to Good Ol' Mr. Lucy and chooses not to do it, only to succumb to temptation later and seek Professor Pitchfork out, but when it comes right down to it, Bob just can't go through with it after all.
Lampshaded: Satan produces 'the standard contract' and remarks that he's been able to fine-tune it over millennia.
Invoked: Satan has been using his influence to nudge Bob into a position where he is open to making a deal in exchange for his soul.
Exploited: Bob is somehow aware of the fact that Satan buys souls, and thus sells his to Satan, only to tell Satan that in exchange he wants Satan to never buy another soul, or something else benefitting the cause of good.
Bob is devoutly religious and refuses to consider the possibility of dealing with Satan.
Alternatively, Satan refuses to make the deal, telling Bob that if he's willing to make a deal with the devil, he'll probably be going to hell anyways.
Discussed: Satan remarks that there's been a drop-off in people approaching him to make deals for their souls.
Conversed: "You'd think that people would suspect that making a literal deal with the Devil is going to end badly and be a bit more hesitant, but nope..."
Bob is incredibly sympathetic, and sells his soul to save his dying wife Alice, despite knowing the cost to himself... but without a soul, he's a monster who kills her anyway.
Alternatively, Satan's obsession with pointlessly backstabbing everyone he deals with means that over time fewer and fewer people are willing to enter into deals with him, until he eventually runs out of the souls he needs.
Reconstructed: Bob sells his soul to save his dying wife, Alice. Without a soul, Bob becomes a monster for a moment and attempts to strangle Alice. However, his soul's good nature wins through and fights it's way free of the devil's grip and back to him. This earns the Devil's ire, and Bob has to deal with a curse of some sort - but he's happy as long as Alice is alive.
Played For Laughs: After the deal, Satan takes pleasure in ensuring that Bob's deal backfires in as humourous a fashion as possible.
Played For Drama: Bob is gradually corrupted by his deal with Satan, being reduced to ever more vile acts of depravity and forcing Alice to find a way to try and void the contract with Satan.