why does Michael's wife accept the sale?

Although the wife, named Susan, begs her husband not to sell the family, Michael ignores her. A sailor soon speaks from the doorway. He will take Susan and Elizabeth-Jane (the child) for five pounds and five shillings--though he will not if Susan is unwilling. Susan sweeps out of the tent with Elizabeth-Jane and the sailor, cursing her husband. 

While Michael announces the auction for the first time, a sparrow flies into the tent, and everyone watches until the sparrow is able to fly away. The sparrow serves as a metaphor for Susan's plight. She is seen as a sparrow: her husband frequently dismisses her complaints as "bird-like chirpings," and everyone in the furmity tent is forced to watch until Susan can escape the tent with the sailor.