What happens at the end of "Araby" when the boy is at the bazaar?
For the narrator, there almost seems to be something magical about Mangan's sister. He describes her "figure [as] defined by the light from the half-open door," while her "dress swung as she moved her body, and the soft rope of her hair tossed from side to side." It almost feels like time slows down for him as he watches her move because he describes it in such detail. She is often defined by light. He has never spoken to her, "and yet her name was like a summons to all [his] foolish blood." The narrator even seems to think of himself as a kind of hero, imagining that he "bore his chalice safely through a throng of foes" when he is really only carrying his aunt's purchases, and Mangan's sister's name "sprang to [his] lips at moments in strange prayers and praises which [he] did not understand." He would cry, for no reason that he could tell, when he thought of her, and he felt "confused adoration." In short, the narrator thinks of her as special, thinks of his feelings for her as special, and thinks of their story as special. When they finally speak and she mentions the Araby bazaar, he thinks of it as special too.
However, when he arrives there—late because his uncle was late to come home, later still because the train was delayed and then moved slowly—he realizes that it is not special, that it sells the same "porcelain vases and flowered tea-sets" that one could purchase just about anywhere else. He listens to the cheaply flirtatious conversation of the young woman and men at the stall and hears a "voice call from one end of the gallery that the light was out. The upper part of the hall was now completely dark." The symbolic light that had once lit up Mangan's sister is gone now, replaced by the darkness. The narrator realizes that his feelings were not special, and that the world will not stop or even care about his feelings (symbolized by his uncle's forgetfulness of how important the bazaar was to him, of the late train). He had seemed to believe that he was special and that he would be able to find something brilliant and unique for his special girl, but he realizes that this was mere vanity.
I can understand your confusion--this story does have an ambiguous ending.
There is a summary of this story at: http://www.enotes.com/araby/summary. This may help you.
What happens at the end of "Araby" is what James called an "epiphany"--a flash of internal revelation or understanding. After the young boy has revered and almost worshiped Mangan's sister, he overhears a flirtatious and shallow conversation at the bazaar between a young woman and two young men. The narrator tells us that he does not purchase anything for Mangan's sister and that "Gazing up into the darkness I saw myself as a creature driven and derided by vanity; and my eyes burned with anguish and anger."
This moment at the fair has made the boy view his feelings differently--he sees his conversation with Mangan's sister as equally silly and shallow, and realizes that she probably did not take him seriously. It is a "growing-up moment" for this young boy.