Charisma affects the prices adventurers pays for good and services from vendors like Marcos Cavielli and Hokas Tokas. For example, a set of leather armor at Marcos's shop has a base price of 100 gold pieces, but this price will be higher for an adventurer with low charisma.
In the same fashion, charisma affects how much gold adventurers receive from Sam Slicker for the treasure they collect in adventures.
Friendship and hostility
Charisma affects the likelihood of a monster being either friendly or hostile. Monsters are more likely to be friendly to a character with high charisma, and more likely to be hostile to one with a low score. The friendliness rating of a monster is also a factor in determining how it reacts: if it has a rating of 100 it will be friendly regardless of the adventurer's charisma, and if it has 0 it will always be hostile.