Here's the list of masks and people you are supposed to sell them to.
Keaton Mask: Guard to Goron City
Skull Mask: Skull Boy in Lost Woods
Spooky Mask: Boy in Graveyard
Bunny Hood: Marathon Runner in Hyrule Field
Goron Mask: Can't sell
Zora Mask: Can't sell
Gerudo Mask: Can't sell
Mask of Truth: Keep this one!
