The traveling merchant is a recurring type of a character in the Final Fantasy series, selling the party supplies during their adventures allowing them to replenish their inventory. They usually follow the party across their adventures, or are already present in the area the party arrives to.
In the Game Boy Advance and subsequent versions, the Traveling Merchant can be met in the Sealed Temple. Afterward he can be found throughout the world in equipment shops and he will sell the player ammunition for the Cannoneer job.
The player will meet Stiltzkin numerous times. He will always have things to sell, and will offer to sell the party three items for a set price.
O'aka XXIII is an itinerant merchant who frequently appears around Spira to sell items. He sells items, weapons, and armor. He returns in Final Fantasy X-2 as a salesman, but no longer travels the world popping up in different locations.
The Traveling Merchant only appears after the party reaches Final Town. Afterwards, he'll be found loitering inside the Item Shops found across the different worlds.