Why does Mr. Avery say he can't buy the items on the shopping list from Vicksburg anymore?

 Mr. Avery says that he cannot buy things in Vicksburg anymore because Mr. Granger and Mr. Montier have asked their tenant farmers for sixty rather than fifty percent of their cotton and has threatened to kick those who shop in Vicksburg off their land. The Wallaces have threatened to get the sheriff to put those who owe debts on the chain gang.