The following is based on Fallout cut content and has not been confirmed by canon sources.
“We have been plagued by Rad Scorpions for some time now. All who have been sent to destroy these monsters have not returned. I will give you weapons and armor if you will follow my guide to their lair and kill them all.”
The Barterman is a trader who was originally intended to initiate the Stop the Radscorpions quest by sending the Vault Dweller off with Sammy as a guide to their location, in hopes he could destroy them. He was cut from the game and Aradesh apparently replaced him in his role.