(Assuming that traditional rules apply and Satan can grant everything.)
A military general who is obsessed with advancing his career, and lives in a country currently on war. (Is the setting the real world?)
A scientist wants to sell his soul in exchange for a way to create clean, cheap, and easy to produce energy.
A priest plans to trick Satan by pretending that he wants a contract, learn as much as he can about how Satan works and then back off without signing the contract at the last minute.
A mass murderer or something similar believes that his soul is already beyond saving, so he might as well make some profit by selling it.