Amongst the Cosmos/Mysterious Merchant

The Mysterious Merchant is an NPC vendor that randomly spawns on the Surface during daytime. He will not move into a vacant house if one is available, similar to the Traveling Merchant, Skeleton Merchant and the Old Man. He cannot take damage.

Quotes

 * "Ah, welcome traveller. Care to purchase my offerings?"

During Nighttime

 * "Ah, traveller. Forgive me, I can only offer you my goods during the day."

Talk

 * "Your questions lie elsewhere, Traveller. Why waste such precious time conversing with a merchant, ha ha ha."