The apothecary was actually the nephew of the man who ran the herbal remedy stall at the Renaissance fair in the Season 6 episode Knight Fall. He was portrayed by actor Ryan Radis.
House and Thirteen went to do another environmental scan of the fair in order to try to find the source of Sir William's illness. House went into the tent and asked the apothecary what some of the things were. However, it quickly became obvious that the apothecary didn't know and House did. House saw something and the apothecary identified it as wild carrot. However, House recognized it as hemlock. He asked if the apothecary had sold any to Sir William, but the apothecary said the only person who had bought some was King Miles. Suspicion immediately fell on Miles as the team was aware his fiance Queen Shannon had feelings for William.