This proverb exposes a common form of dishonesty: saying something is worthless in order to get it cheaply, then boasting about the deal afterward. The hypocrisy is clear — you never believed it was worthless. Dishonesty in business dealings is a form of theft, even if it is technically legal.
Memory phrase: Be honest in every deal.
Talk about it: Why is it wrong to say something is bad just to pay less for it?
Prayer: God, help me be honest even when I could get more by lying.