The Poe Salesman is a character from Ocarina of Time. He lives in the room at the bridge of Hyrule Castle, sitting on a bed with a stick and a cage of poes suspended above him. Despite his title, he simply seems to purchase poes from Link, not sell them. He will buy regular poes for 10 rupees.
The main purpose for this character was to provide a challenge for an empty bottle. He wanted a custumor to hunt ten Big poes. Even if Link does not fully complete this challenge, he still earns 50 rupees for every poe he brings to the salesman.