Go to the Waterfall Cave in Skyloft at Night. Search for 3 Blessed Butterflies flying around and play your harp like you would to summon a Gossip Stone. The Moonlight Merchant will be summoned instead. He sells rare treasures, which are expensive, but hard to find any other way.

Item	Location
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