Keaton Mask
Sell it to the guard at the Death Mountain gate in Kakariko Village. If used in the "beauty contest" in the Lost Woods, the Deku kids will spit nuts at you.
Skull Mask
Sell it to the lone Skull Kid on the left from the entrance to the Lost Woods. If used in the "beauty contest" in the Lost Woods, the Deku kids will say you look like their "totem" and give you the ability to carry a larger amount of Deku Sticks. Keese will leave you alone when you wear it.
Spooky Mask
Sell it to the kid in the graveyard. If used in the "beauty contest" in the Lost Woods, the Deku kids will give you rupees.
Bunny Hood
Sell it to the man running in Hyrule Field at night after you get three spiritual stones. If used in the "beauty contest" in the Lost Woods, the Deku kids will give you rupees. Stalkids will leave you alone when you wear it.
Zora Mask
It looks like a Zora, and is free to borrow after selling all the previously mentioned masks. If used in the "beauty contest" in the Lost Woods, the Deku kids will spit nuts at you. You can get some interesting reactions from the Zoras with this mask on.
Goron Mask
It looks like a Goron, and is free to borrow on the same conditions as the Zora Mask. If used in the "beauty contest" in the Lost Woods, the Deku kids will give you rupees. You can get some interesting reactions from the Gorans with this mask on.
Gerudo Mask
It looks like a Gerudo, and is also free to borrow as the Goron and Zora Masks are. If used in the "beauty contest" in the Lost Woods, the Deku kids will spit nuts at you. You can get some interesting reactions from the Gerudos with this mask on.
Mask of Truth
It allows you to talk to the gossip stones and is free to borrow after having sold the first masks listed. If used in the "beauty contest" in the Lost Woods, the Deku kids will give you the ability to carry the largest amount of Deku Nuts.