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.
The "two men from the states" come along to buy the harness, team and all.
As you continue in the chapter (Chapter 5), you find out the names of the two men are "Hal" and "Charles," and they appear to be completely without experience in the harsh environment of "The North." Their description very clearly foreshadows the problems to come as they attempt to make a journey they are in no way prepared for.