Where To Sell The Masks:

Here's a list of the masks and the people who will buy them from you.

Mask of Truth -- Keep this one!
Keaton Mask -- Guard in Kakiriko Village
Skull Mask -- Skull Boy in Lost Woods
Spooky Mask -- Boy in Graveyard
Bunny Hood -- Marathon Runner in Hyrule Field
Goron Mask -- Can't sell to anyone
Zora Mask -- Can't sell to anyone
Gerudo Mask -- Can't sell to anyone