Most people in my game don't use charisma checks, so I just put it down as a diplomacy check untrained or not.
when an Npc and a PC negotiate, it's opposed roll, if the pc's roll high it can shift the NPC's attitude towards friendly, and if the PC rolls low the NPC might not budge from his position or turn hostile.
works great so far.
when an Npc and a PC negotiate, it's opposed roll, if the pc's roll high it can shift the NPC's attitude towards friendly, and if the PC rolls low the NPC might not budge from his position or turn hostile.

works great so far.