Jamieson, Fausset and Brown on Amos 8:6

6. buy ... poor for silver ... pair of shoes--that is, that we may compel the needy for money, or any other thing of however little worth, to sell themselves to us as bondmen, in defiance of Le 25:39; the very thing which brings down God's judgment (Am 2:6).

sell the refuse of ... wheat--which contains no nutriment, but which the poor eat at a low price, being unable to pay for flour.