At night, go to the Waterfall Cave in Skyloft. Find the three Blessed Butterflies nearby play your harp as if summoning a Gossip Stone. The Moonlight Merchant will appear. The following items can be purchased from him.

Blue Bird Feather: 200 Rupees
Dusk Relic: 100 Rupees
Evil Crystal: 100 Rupees
Goddess Plume: 200 Rupees
Golden Skull: 200 Rupees
Monster Horn: 100 Rupees