Jamieson, Fausset and Brown on Amos 8:5

5. So greedy are they of unjust gain that they cannot spare a single day, however sacred, from pursuing it. They are strangers to God and enemies to themselves, who love market days better than sabbath days; and they who have lost piety will not long keep honesty. The new moons (Nu 10:10) and sabbaths were to be kept without working or trading (Ne 10:31).

set forth wheat--literally, "open out" stores of wheat for sale.

ephah--containing three seahs, or above three pecks.

making ... small--making it below the just weight to purchasers.

shekel great--taking from purchasers a greater weight of money than was due. Shekels used to be weighed out in payments (Ge 23:16). Thus they committed a double fraud against the law (De 25:13, 14).