The opening paragraph of Jack London's novella, The Call of the Wild provides the answer to why Manuel steals Buck and sells him:
...Because men, groping in the Arctic darkness, had found a yellow metal,...These men wanted dogs, and the dogs they wanted were dogs...with strong muscles by which to toil, and furry coats to protect them....
It is the time of the Gold Rush, the Klondike strike. And, Manuel, the gardener, is in debt because of his uncontrollable gambling. Since he does not make enough money to provide for his wife and many children and also play Chinese lottery, Manuel has put himself in debt. So, in order to procure money with which to pay his debts, he takes Buck off one night while the Judge and others of his family are gone. Having prearranged for the new owner to meet him, Manuel hands off Buck for money.
Manuel kidnaps Buck because he want to make money out of him by selling him. Manuel needs this money to fund his gambling habit, and also he has a large family to provide for. We can also assume that he probably doesn't get paid a great deal for his work as a gardner's helper on the wealthy estate where Buck lives. Buck is a very valuable dog and there is good money being paid on the black market for strong dogs to pull sleds in the cold north, where gold is being mined. It is not likely that Buck would ever have left his comfortable life as an adored family pet in a rich household had it not been for Manuel.