The Man from the Trading Post works at his shop during the day. For most of the night, he has a Part-time Employee fill in for him. The man will sell his products to any of Link's three forms except Deku Link, whom he treats like a child. At all times, the man appears to be in high spirits and is very knowledgeable about his products.
In Majora's Mask 3D, the Ocean and Swamp Fishing Hole Owners will deny that they have any relation to and/or knowledge of him, despite their resemblance.
Due to the man's striking resemblance to the Man from the Curiosity Shop, it is believed that the two men are one and the same. Further evidence supporting this theory includes the Part-time Employee's rambling curiosity about the Trading Post's strangely high amount of Rupees, considering business rates, though this can be interpreted in other ways. Also, if Link talks to him while wearing the stone mask, he will say to himself that he has to get ready for the night shift, however, the night shift is run by the Part-time Employee and starts at the same time as the Curiosity Shop opens, implying that his night shift is actually held in the Curiosity Shop. If true, it would also imply that the man wears a toupee.
If Link talks to the Man from the Trading Post while wearing Kafei's Mask, he first recognizes "Kafei", but immediately corrects himself, insistently claiming he does not know who he is. However, he refers to Kafei as a kid, and everyone except the Man from the Curiosity Shop (and possibly Mayor Dotour) would refer to Kafei as an adult, not knowing he had been transformed into a child, which leads to the possibility that the Man from the Trading Post knows about Kafei's situation.
Although this is possible, there are reasons this may not be true. For example, talking to this man does not open the Man from the Curiosity Shop's entry in the Bombers' Notebook, and by using the hole in Kafei's hideout, it is possible to see the Man from the Curiosity Shop at his desk while the Man from the Trading Post is still working. Nonetheless, there is a chance both of these examples are simply errors made in development, though the former could have been intended to be a mistake on Link's part.
It should be noted that the official Nintendo Power strategy guide states that the two are the same person.