In the Legend of Zelda: Ocarina of Time, the Happy Mask Salesman side quest involves selling four different masks to different people around Hyrule and completing the quest gives the Mask of Truth, which enables the player to talk to Gossip Stones and get valuable tips and information. The player can also collect more Rupees and receive Deku Stick Bag upgrades.
How to Start the Happy Mask Salesman Quest

Travel to Castle Town and find the Happy Mask Salesman’s shop to the right of the fountain. If you arrive during the night, simply play the Sun’s Song to transition to daytime.

Enquire at the shop, and the owner would gladly offer you the first mask free of charge. Your task will involve selling the mask and returning to him with the money.
How to Sell All Four Masks
1. Sell the Keaton Mask to the Guard

Put on the Keaton Mask and make your way to Kakariko Village through Hyrule Field. Approach Death Mountain and locate the guard at the gate.

Once you have worn the mask, converse with him. He will buy it from you for 10 Rupees, which you are required to give to the Happy Mask Salesman to access the Skull Mask.
2. Sell the Skull Mask to the Skull Kid

Prior to selling the Skull Mask, utilize it to earn the largest Deku Stick Bag upgrade in the Lost Woods.
Travel to the Lost Woods, north of Kokiri Forest, and follow this route: right, left, right, left, and left again. Look for a hidden hole among the bushes and drop into it.

Put on your Skull Mask and step onto the platform, the Deku Scrubs will gather around you and wait in line before a bigger Deku Scrub. Talk to the bigger one to claim the larger Deku Stick Bag upgrade.

Afterward, go back to the entrance of the Lost Woods and take the first left to find a Skull Kid on a stump. Before selling the mask you should play Saria’s Song while standing on the stump to receive a Piece of Heart.

Then, wear the Skull Mask and talk to the Skull Kid who will give you only 10 Rupees, even though the mask costs 20 Rupees. Return to the salesman and pay the full 20 Rupees for the Spooky Mask.
3. Sell the Spooky Mask to the Graveyard Child

With the Spooky Mask in hand, head to the graveyard in Kakariko Village. It is important to visit during the daytime since the child you need to see wanders only during the daytime.

Put on the mask and talk to him. He will pay 30 Rupees for the mask. Next, return to the Happy Mask Salesman and give him the rupees in order to receive the Bunny Hood.
In case you still haven’t learned the Sun’s Song, you can find it at the Royal Tomb located in the back of the graveyard. Go through the hidden cave in order to learn this song.
4. Sell the Bunny Hood to the Running Man

The first step is to clear the third dungeon, Inside Jabu-Jabu’s Belly, which will earn you the third Spiritual Stone.
Once this is accomplished, a running man will show himself somewhere around Hyrule Field. You can either wait outside Hyrule Castle for him to show or simply look for him around the area.

Put the Bunny Hood on and talk to the running man. He will give you a Gold Rupee worth 200 Rupee.
Once again head back to the Happy Mask Salesman and pay the remaining amount of 50 Rupees to complete the mask-selling portion of the quest.
How to Get the Mask of Truth

Once you pay back the necessary money for all four of the masks, chat with the Happy Mask Man who will congratulate you and reward you with the Mask of Truth.
In addition, you will have all of the remaining masks in his collection, like the Zora Mask and Goron Mask, unlocked. You can freely exchange masks without paying anything extra.
What Does the Mask of Truth Do?
The Mask of Truth provides its user the option to get in touch with the Gossip Stones which are often referred to as Sheikah Stones and are present at various locations across Hyrule.
Wear the mask and speak to these stones to discover useful hints, tips, and lore. You can find some outside the Happy Mask Salesman’s shop and along the stairs leading towards the Temple of Time.
A useful tip offered by one of the Gossip Stones is that the use of Ocarina in front of the stone helps activate a fairy ability that automatically restores your hearts. Without putting on the Mask of Truth, it is generally impossible to receive these messages.

