“Unequal weights.” The Hebrew text literally says, “a stone and a stone” are an abomination to Yahweh. That would be clear to people reading Proverbs at the time it was written. At that time, most of the weights that merchants used in buying and selling were made of stone, not metal, and unscrupulous merchants often kept stones of different weight in their bag that only they could easily tell apart so that they bought a lot and sold a little. Thus, for example, a merchant may have had two supposedly 5-shekel weights in his bag, but they were actually a little different in weight even though they looked the same and felt the same to the untrained hand. But the merchant could tell them apart and he would buy with the heavier weight to buy more, and sell with the lighter weight to sell less. But that kind of dishonest dealing is an abomination to Yahweh (Lev. 19:35; Deut. 25:13-16). [For more on trading using honest balances, see commentary on Prov. 11:1].
“are not good.” This is the figure of speech, tapeinosis, “understatement.” False scales are not just “not good,” like unequal weights they are an abomination to God.