The Happy Mask Shop in Hyrule Castle Town lets you borrow masks to sell. The more you sell, the more masks you unlock.
The Happy Mask Shop
You can visit the Happy Mask Shop in Hyrule Castle Town after you have shown the letter to the soldier in Kakariko Village next to the entrance to Death Mountain Trail.
Borrow the Keaton Mask from the Happy Mask Shop, then wear it while talking to the soldier in Kakariko who unlocked the path to Death Mountain Trail for you. He will buy it.
You can get the Skull Mask after selling the Keaton Mask. Go into the Lost Woods and go left from the entrance to find the Skull Kid playing the flute. Wear the Skull Mask and talk to the Skull Kid to sell it to him.
Next, you can get the Spooky Mask from the shop. Wear it and talk to the boy in Kakariko Graveyard. He is there during the day. The boy will buy the mask from you.
You can only sell the Bunny Hood after you have completed the Lord Jabu-Jabu dungeon. Go to Hyrule Field during the day and look for the man running around Lon Lon Ranch. At night you can find him sitting near the entrance to Gerudo Desert. Wear the mask and he will buy it from you.
After you sell the Bunny Hood, there are no more masks to sell, but you can borrow the Mask of Truth, Gerudo Mask, Goron Mask, and Zora Mask after selling the Bunny Hood.