Keaton Mask — Borrow from the Happy Mask Shop once Zelda's Letter opens it. Sell it to the guard at the Kakariko gate for 15 Rupees (he pays 10, you owe 10).
Skull Mask — Second mask on loan. The Skull Kid in the Lost Woods (first left turn) buys it for 10 Rupees, less than the 20 you owe.
Spooky Mask — Third mask. The boy wandering the Graveyard by day buys it for 30 Rupees.
Bunny Hood — Fourth mask. The Running Man jogging around Hyrule Field pays 500 Rupees for it (you receive the sale as an overflow of your wallet). On 3DS it also makes you run faster.
Mask of Truth — After settling the Bunny Hood debt you can borrow the Mask of Truth (plus the Goron, Zora and Gerudo masks). Lets you read Gossip Stones and unlocks a Deku Theater reward.