“differing weights.” The literal Hebrew is “a stone and a stone.” This command was understood in the Old Testament culture. The command is not saying a traveling merchant cannot have different weights in his bag, i.e., a one-shekel weight, a five-shekel weight, a 20-shekel weight, etc. What it is forbidding is having differing weights but claiming they are the same weight. Unscrupulous merchants often kept stones of different weight in their bag that only they could easily tell apart; stones that were a little heavier for buying and stones that were a little lighter for selling, so that they bought a lot and sold a little. But that kind of dishonest dealing is an abomination to Yahweh (Lev. 19:35). Cheating in buying and selling has been around for thousands of years, but God sees and will repay. It is better to have a little less in this life and more in the next than more in this short life but less in the next.
[For more on trading using honest balances, see commentary on Prov. 11:1.]