The narrator made the decision to buy something at the bazaar, "Araby," for Mangan's sister. The narrator has a crush on Mangan's sister; he essentially becomes obsessed with her, thinking about her at all moments. When he was not near her, he imagined himself like a knight carrying out a mission for his princess. So, when he tells her that he will bring her something from the bazaar, it becomes his mission. Since "Araby" has connotations of the East/Middle East, the narrator has an even more heightened sense that he's going to some exotic land to retrieve a prize for Mangan's sister.
The syllables of the word Araby were called to me through the silence in which my soul luxuriated and cast an Eastern enchantment over me.
It is likely that he intended to buy something exotic looking, something out of the ordinary. He looks at porcelain vases, tea sets, and jars that "stood like eastern guards at either side of the dark entrance." The narrator becomes disheartened and loses interest. It is unclear what he intended to buy and it is unclear if he even found anything that he liked because he becomes disillusioned with the entire experience.