The Poison Salesman is a non-player character who can be found near the bar in Seers' Village, and is involved in The Fremennik Trials and Murder Mystery quests.
The Poison Salesman is involved late in the Murder Mystery quest. At a certain point, the player will travel to Seers' Village pub and ask him about the poison that was used in the murder. He is not very helpful, however, and tells the player that all the party guests purchased poison from him.
During Manni the Reveller's trial, the player will be required to visit the Poison Salesman and purchase a keg of low-alcohol beer for 250 coins.