Why did Yosef not accept their answer?
Rashi #1: He claimed that their explanation 1 was unacceptable.
Rashi #2: After ascertaining that if they found their lost brother, they would pay any ransom that was asked of them, Yosef asked them what they would do if his master would refuse to sell him, to which they replied that they were willing to kill or to be killed on his behalf. That was when he reiterated his accusation. 2
Seforno: He claimed that the 'missing' brother, whose location they refused to divulge, had gone back to report to their father what they had seen, or that they had decided to spy out the land.
רש"י: הדבר אשר דברתי וכו': ממה בא רש"י להוציא, ומה כוונתו?
גור אריה: בפשטות משמע מהפסוק שיוסף מביא ראיה מדבריהם לכך שהם מרגלים, ואין הדבר כך אלא כאילו כתוב 'הדבר אשר דברתי, הוא אמת ונכון'.