The Legend of Zelda: Ocarina of Time Unlockables

Masks Sidequest
This sidequest becomes available as a child after showing Zelda's Letter to the Death Mountain guard. Masks are picked up at the Happy Mask shop in Hyrule Castle Market and must be sold to NPCs. To see if someone wants to buy a mask just wear it when talking to people.

Successfully selling all four of these masks will grant you access to the Mask of Truth, Goron mask, Zora mask and Gerudo mask. Note that the last three don't have any special effects. Also, earlier masks will show up in the shop again after giving the money for them to the shop.
MaskWho to sell it to
Keaton MaskSell to the guard at Death Mountain Trail entrance in Kakariko Village. Give 10 rupees back to the shop.
Skull MaskSell to the lone Skull Kid in the Lost Woods. Give 20 rupees back to the shop.
Spooky MaskSell to the kid in Kakariko Graveyard during the day. Give 30 rupees back to the shop.
Bunny HoodSell to the man running around Hyrule Field (you have to wait for him to stop and take a break). Give 50 rupees back to the shop.