In the Call of the Wild, what motivates Buck's being sold as a sled dog?

The Call of the Wild was written by Jack London and published in 1903.

The main character in the story is Buck, a dog mixture between a Saint Bernard and a Scotch Collie.

In the early stages of the book, Buck is a cherished domesticated dog. He lived in a comfortable house in Santa Clara, California.

Unfortunately, one of the family's gardeners where Buck lived stole the dog to be sold afterward. This was due to the fact that gold had been discovered in Yukon, Canada and the demand for sled dogs had increased.

The gardener sold him to dog traders as a way of gaining money. Buck was obliged to work as a sled dog for two government couriers.