why was the team sold

Depends on where you are in the book. Early in the book, the team is sold because Francois and Perrault no longer have a need for their services because they do not have a mail contract. After the Scotsman buys them, they come into town bedraggled and underfed. Charles, Mercedes, and Hal buy them to take off toward the gold strikes with absolutely no idea what they are getting into. When they run into John Thornton, he releases Buck from their care, threatening to kill Hal. The team then falls into the ice and all except Buck die, including the adults.

Also, Thorton stays with buck and lives