Why does she lie to her client about where the bowl had been bought? How does the author build suspense into the story?

Initially, the real estate agent feigns ignorance about the bowl but eventually tells her client that the bowl had been a present. Furthermore, she maintains that the owners of the home had no idea where the bowl had been purchased. 
The real estate agent lied because she does not want her client to know that the bowl actually belongs to her. Furthermore, telling the client the truth would have exposed the agent's use of the bowl to manipulate the emotions of potential buyers. As the narrative reveals, the agent typically places the bowl in strategic places during each house viewing.
As for your other question, the author builds suspense by introducing a dilemma part way into the story, one that concerns the agent's relationship with the bowl. Essentially, the real estate agent becomes obsessed with the bowl and its contribution to her real estate business success. She describes the depth of her emotional distress when she unintentionally left the bowl behind during one of her property showings. In the agent's mind, the bowl had come to represent a sort of magic talisman that ensured her continued business success.
Thus, the author builds suspense by adding a sense of apprehension or the feeling of impending peril to the story. The bowl morphs into a quasi-mystical object, with a personality of its own. At the height of her paranoia, the agent wonders if the bowl feels slighted in some way. She begins to imagine herself a villain who has denied the bowl its just rewards for all the good luck it has brought her. The agent becomes possessive, much like a woman who suspects that her lover is planning to leave her. By this point in the story, we begin to question whether the agent harbors schizophrenic tendencies or whether the bowl will break in some tragic accident.
Essentially, the author uses this sense of impending peril to build suspense in her narrative. Interestingly, the story ends ambiguously, with the author pointing out that the agent often spends quiet moments merely staring at the bowl.