The answer to this can be found in Chapter 5 of the book. The men who buy the team are Hal and Charles. When they buy the team, it marks the beginning of the low point for Buck.
Hal and Charles and Mercedes (Charles' wife and Hal's sister) have no idea as to how to care for the dogs or how to use them. Because of this, they soon wear the dogs out and some of them die.
John Thornton sees this when they stop at his camp. When they start beating Buck one morning, he takes Buck forcibly. This leads to a close bond between man and dog.