Who is the character that sells Buck to pay off debts?

The character who sells Buck to pay off debts is Manuel. Manuel is a servant in the household of Buck's owner, Judge Miller. Facing financial troubles, Manuel takes advantage of Buck's value and sells him as a means to relieve his debts. This act sets off the chain of events that lead Buck into the brutal life of an Alaskan sled dog, where he must adapt to new challenges in a harsh wilderness.

The other characters listed do not engage in selling Buck for this purpose. Thornton is a compassionate character who later cares for Buck after his harsh experiences. Matthewson is a buyer at the dog auction but does not sell Buck; he is more of a representative of the competitive pressures Buck faces after being sold. The man in the red sweater is significant in Buck's life because he represents the brutality of the human world Buck is thrust into after becoming a sled dog, but he is not the one who sells Buck.