Kupfer was brutally stabbed shortly after she asked one of her managers to call her because someone was in the store "giving me weird vibes."
By the time the manager saw the message, just four minutes later, Brianna was already dying on the floor of the store, a prosecutor said Thursday.
The prosecution says Kupfer was stabbed 46 times by Shawn Laval Smith, now 34 years old.
Kupfer's family stepped out of the courtroom when they played an audio recording of what was described as the last seven minutes of her life.
The deputy district attorney says Smith made the recording of his interactions with Kupfer and left the device behind when he fled.
In the recording, a man asks Kupfer questions about the store and furniture, leaves momentarily and returns. After a brief conversation, he tells Kupfer he is not going to hurt her. He tells her to get down on the floor. Then Kupfer's screams can be heard.
Several witnesses took the stand including responding officers and the woman who found Kupfer's body minutes later and called 911.
The prosecution says DNA that matches Smith was found on the knife handle with Brianna's blood. Smith was arrested days later in Pasadena.
If convicted on all counts, he faces a maximum sentence up to life in prison with no parole.