The Happy Mask Salesman is a recurring character from The Legend of Zelda series, as his name implies, he is greatly attributed to masks and apparantly knows a great deal about individual masks.
The Legend of Zelda: Ocarina of TimeEdit
He runs the Happy Mask Shop, where Link can borrow masks to sell to other people, and give The Happy Mask Salesman the money back, so he can sell a new mask. This creates a small trading sidequest for rupees and a special mask
Now, the Salesman is more a art of the main quest. He was the previous owner of the Majora's Mask, until it was stolen by Skull Kid. Now, the salesman is desperate to get it back to the point of asking/threatening Link to claim it in his stead. He helps Link break a curse that Skull Kid put on him, requiring Link to reclaim his Ocarina. He had hoped that Link would claim the mask at the same time. When this did not occur, the salesman went into a frightening psychotic episodes, warning Link of the mask's destructive, foreboding power. He waits in the Clock tower for Link to return with the mask and save Termina from Majora's moon.