The Question and Answer section for Call of the Wild is a great resource to ask questions, find answers, and discuss the novel.
That is because Buck is dognapped from pampered civilization and sent to the primitive North. Also Buck is discovering his primitive instincts dealing with his abusive captors.
The men that buy buck are just called strangers. I think one is later known as simply "the man in the red sweater".
Buck trusted the men he knew so he accepted the rope from Manuel the gardener before being stolen from his home.