Nicolas is found next to his house, to the left upon entering High Hall in the Orobas Fjords. He is a merchant, selling a small selection of weapons, armor, potions, and miscellaneous items (gems, ores, herbs, etc.). His wares includes the Hunter Bow, part of the Hunter set.
After dealing with the trolls attacking the village, he is also willing to sell Drudanae if asked ("do you have anything special in your stock?"). Required for the quest A Puff of Drudanae (a sub-quest of Down the Hatch), it can be purchased, or acquired free by unlocking the cellar in his house which leads to the cliff where he cultivates his Drudanae. He also had a book concerning the herb (Drudanae, a Manual) on his table.
Mindreading him lowers his prices, makes the cellar key available to loot, and reveals: "Is hiding a key under your pillow a good idea? Yeah, nobody will look there. My cellar is safe."