Keaton Mask - Guard - Kakariko Village
The Guard wanted a mask for his son, and Link delivered. You'll also pick up 15 rupees.

Skull Mask - Skull Kid - Lost Woods
The kid says it makes him look tougher. You dont make a profit from this one.

Spooky Mask - Kid - Kakariko Graveyard
Give the mask to the wannabe Dampe. You'll get 30 rupees from the kid.

Bunny Hood - Running Man - Hyrule Field
This guy wants to be like a rabbit, and you make a huge profit.

Mask of Truth
After selling the Bunny Hood, the owner of the Happy Mask Shop gives you this mask. It allows you to talk with the stones that usally tell the time.

Other Masks
After you get the Mask of Truth, you can trade it in for the following masks. People will usally respond differently to you when you use these masks in the right places..

Goron Mask - Use in Goron City.
Zora Mask - Use in Zoran Domain
Gerudo Mask - Use in Gerudo Fortress