Peter Potter the Poison Salesman is a merchant who can be found in the Forester's Arms in Seers' Village, and is involved in The Fremennik Trials and Murder Mystery. He is a very enthusiastic salesman who would go great lengths to tell his customers about the benefits for using his products. When he advertises, he frequently uses alliteration (using the same beginning letter in rapid succession).
During Murder Mystery, you have to talk to him regarding the murder of Lord Sinclair. He's located in the pub in Seers' Village. You have to ask him which of Lord Sinclair's children purchased poison. He'll tell you that all of them bought poison from him.
During Fremennik Trials, he sells you some non-alcoholic beer to complete one of your trials.