In the book The Call of The Wild, who bought the team harness and all for a "song"?

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.