Power, money, love are the classic three.
My devils never deal in souls, since most players realize thats a bad thing to give up. I've traded Magic Armor for an Unborn baby once. Recently I gave out a Magic Dagger for a year of someone's memories. Players are unlikely to give up souls for anything, since in a game like this they typically feel that they ARE their soul, so the prize would have to be HUGE, like all the kingdoms of the earth kind of huge.
The movie End of Days had one guy give up his soul in order to not die a horrible flaming death. (Which he later rescinded)
Escape from a terrible situation. If they are about to die, or be tortured, or something similarly bad, selling their soul (as a nebulous future consideration) might feel worthwhile to escape.
Revenge. Some bad guys have just slaughtered your entire family/village/etc, and they sell their soul for power to kill the bad guys.
The main point is for the soul-sellers to be desperate and emotional. They don't have calm and quiet to think it through.
Beauty, healing/ health, station/ status, youth, to restore a lost loved one to life, to cure a sick loved one, to undo a terrible mistake, freedom/ escape, to have a child, to have an heir
An unnatural gift, super power, blessing or magic item: a persuasive tongue, invisibility, cornucopia, to never be hungry, immortality (always with a loophole...), never to be bound, never be defeated in combat, never be tired, to have the strength of a lion, to be able to assume the form of any living creature, to speak the tongues of birds, to know every word ever written in a certain language etc.