Here is where to trade all of the masks from the Happy Mask Shop.

Keaton Mask: Royal Guard at the edge of Kakariko Village on the way up to Death Mountain. He pays full price, and not a penny less.

Skull Mask: Enter Kokiri Forest and head for the Lost Woods. Go one room to the left. Hop on the tree stump while wearing the mask and play Saria's Song. The Skull Kid will then ask you for the mask. He pays a few rupees short of full price.

Spooky Mask: Sell it to the boy in the Graveyard in Kakariko Village. He also pays full price.

Bunny Mask: Sell it to the man running around in Hyrule Field by Lon Lon Ranch. You must first defeat the Forest Temple as Adult Link and go back in time, the jogger will then be there. He gives you rupees until your total amount is maxed. I recommend having an empty Giant's Wallet!

Mask of Truth: With this mask you can talk to the "Gossip Stones" that have the eye on them and are all over Hyrule. They will reveal helpful hints throughout the game.

Goron Mask, Gerudo Mask, Zora Mask: Fun masks that have no particular purpose in the game. Wear them while talking to members of each race to see their reaction.

By Gaming Freak