Could a common carrier be held liable for the loss of a valuable item stolen by other passenger when the victim told the driver that he has valuable item?

Yes. Ordinarily, the common carrier is not liable for acts of other passengers. But the common carrier cannot relieve itself from liability if the common carrier’s employees could have prevented the act or omission by exercising due diligence. In this case, the passenger asked the driver to keep an eye on the bag which was placed beside the driver’s seat. If the driver exercised due diligence, he could have prevented the loss of the bag. (1997 Bar Question)