Ocarina of Time Masks
In Ocarina of Time, there's a Mask Shop where you can borrow masks, sell them, then return with the money to borrow more masks. If you finish this entire Trading Sequence, you can get some upgrades as well as a cool item that lets you speak with Gossip Stones.
The Happy Mask Shop is located in the Market, right between the Temple of Time and the exit leading to Hyrule Castle. In the Nintendo 64 version of the game, the building has two giant eyes on top of it. The Nintendo 3DS version of the game still has two giant eyes, but it is much more colorful and stands out.
The Mask Shop will remain closed in the early portion of the game and will first open up after you give Zelda's Letter to the Guard at the base of Death Mountain in Kakariko Village.
Keaton Mask
The first mask you're eligible to borrow is the Keaton Mask (10 Rupees), which is a pretty easy one.
The guard at the base of Death Mountain in Kakariko Village spoke of it at length and how he wanted it for his child. Slap it on and talk to him, he'll give you 15 Rupees for the darn thing.
Skull Mask
The second mask is the Skull Mask (20 Rupees), which would be impossible to find based on just what people say...
Go to the Lost Woods in Kokiri Forest. Once there, go left once to find an imp playing the flute. He's kind of an outcast amongst the rest of the imps because he doesn't think highly of himself and his "toughness." Jump onto the stump and show off the Skull Mask. He'll buy it for a measly 10 Rupees. JERK!
Spooky Mask
The third mask looks just like a ReDead (which you'll see soon enough, 30 Rupees). It's also a fairly easy one to do.
Head to the Kakariko Graveyard during the day to find a small kid stomping around with a stick. His life dream is to be like Dampe, the grave keeper and be scary. Somebody needs a hobby... Anyway, flash him the Spooky Mask and he'll cough up all 30 Rupees without blinking. Sheesh!
Bunny Hood
This last mask isn't as obvious... The Bunny Hood (50 Rupees) is simple, but will require you to run around.
In Hyrule Field, there's a guy known as the Running Man who is constantly jogging. He will only appear after Link has gathered all three spiritual stones. He starts out in the northeast corner and works his way around the field counter-clockwise, sticking close to the road surrounding Lon Lon Ranch. The easiest way to find him is to enter Hyrule Field from the Market and follow the road leading right. By the time you walk past Lon Lon Ranch, you'll see him.
Wait until he stops to take a breather (play the Sun's Song) and talk with him while wearing the Bunny Hood. He's heard stories of it giving the one who wears it mythical speed and agility. Fork it over and he'll pay you over 500 Rupees! It's a real shame, because that's more than your wallet will allow... If you wait for him to get up, you'll see him zip away crazy fast! Now why couldn't it do that for us? If you want to see that, you'll have to play the sequel to this game. =)
The Mask of Truth
After you've brought back the final mask, you'll get four new masks to try out. Three of them are just for fun, but the one of interest is the Mask of Truth, which allows you to read the minds of animals and people you talk to. It also lets you hear the thoughts of Gossip Stones, the giggling, exploding stones across Hyrule that tell you the time.