Quote 1
Perrault took a hand. Between them they ran him about for the better part of an hour. They threw clubs at him. He dodged. They cursed him, and his fathers and mothers before him, and all his seed to come after him down to the remotest generation, and every hair on his body and drop of blood in his veins; and he answered curse with snarl and kept out of their reach. He did not try to run away, but retreated around and around the camp, advertising plainly that when his desire was met, he would come in and be good. (4.10)
Buck can match the men not only in strength, but also in mental prowess. He communicates with them, making his determination and desires clear.
Quote 2
Perrault grinned. Considering that the price of dogs had been boomed skyward by the unwonted demand, it was not an unfair sum for so fine an animal. The Canadian Government would be no loser, nor would its dispatches travel the slower. Perrault knew dogs, and when he looked at Buck he knew that he was one in a thousand-- "One in ten t'ousand," he commented mentally. (1.45)
Men who are well educated in the way of dogs can better recognize how valuable Buck is.