Man from Curiosity Shop

Curiosity Shop Guy
This is the man from the Curiosity Shop who lives in the land of Termina. He sells stolen goods and he usually gets them from a Takkuri or Sakon. If you save the old woman who sells the bombs on the midnight of the first night, you can purchase the All-Night Mask from him. He works during the night. He is also a small part of the Kafei and Anju side quest.

He is a somewhat friendly man and seems to be quite hard working although he becomes sort of uptight and curt if you mock him by way of masks. The function of his shop is vaguely explained but it seems to be a sort of pawn shop combined with a black market (stolen, exotic, and possibly illegal items are often sold there). He, however, claims that he is not guilty of any illegal activity.

He seems to have a strong but sketchy connection with Kafei. Kafei houses in the back of the shop, spying on customers, waiting for Sakon to show up to try to sell the Sun Mask to the Curiosity Shop Guy. The Curiosity Shop Guy doesn't seem to care as he has known Kafei for a long time.

The Curiosity Shop Guy is not very polite. If Link talks to him in either his Goron form or his Zora form, the Curiosity Shop Guy will tell him not to come back. He does not serve people who wear masks.

Naming Note: His actual name is never given but the Bomber's Notebook lists him as "Man from the Curiosity Shop."

Trading Post Guy
The frontman (possibly proprietor) of the Trading Post located in Clock Town in the game The Legend of Zelda: Majora's Mask. After 10PM at night, he has a part-time worker man the Trading Post.

The Trading Post Guy is much more polite than the Curiosity Shop Guy. He will let both Goron Link and Zora Link shop in his store. The Trading Post Guy doesn't serve people who wear masks either.

The Trading Post Guy's part-time worker can often be found clinging to a tree in Termina Field.

Same Person
It is possible that the Curiosity Shop Guy and the Trading Post Guy are the same person. This is because they both use the same model. Their differing personalities can easily be explained by common social phenomena. This means that a pawn shop and a general store have different environments. A pawn shop owner would have to be much tougher with the customers. This is because they are likely to be criminals. A general store owner would have no reason to assume that there will be trouble, so he can be much nicer to his customers.

If indeed they are the same person, the something he says in the Trading Post might indicate why he doesn't like people wearing masks. This might mean that the Skull Kid has done something to him.

On the night of the second day, before 9PM, both the Curiosity Shop Guy and the Trading Post Guy can be seen in their respective stores. If the player has followed the Kafei and Anju sidequest then it is possible to look into the Curiosity Shop from the backroom (Kafei's house). The Curiosity Shop Guy will be seated behind the counter. At the same time, the Trading Post Guy will be in the Trading Post. As convincing as this fact is, there is no way to know if it is just a development error or not.

The Bomber's Notebook does not add the Curiosity Shop Guy to the book when Link talks to the Trading Post Guy. This seems like strong evidence against them being the same person. However, this too, might be a development error.

It may never be proven one way or the other. It is up to you, the reader, to decide for yourself.

Trivia

 * If Link happens to stop by the Curiosity Shop Backroom (where Kafei stays), provided that Kafei has followed Sakon to Ikana Canyon, the Curiosity Shop Guy will tell Link where Kafei went and give Link the Keaton Mask.
 * His Ocarina of Time counterpart is the Itchy Fisherman from the Fishing Pond as a child.