In eight verses, David described another problem in his day that has always been a part of human history: telling lies. One of the ten commandments God gave to Moses was “you must not testify falsely against your neighbor”. Exodus 20:16 (NLT). That was a problem David experienced during his life.
But based on what David wrote, it appears that the lying was for the purpose of financial gain because he referenced violence done to the poor and helpless. For example, a rich farmer could have shown a person fat and healthy animals but, after receiving payment, delivered an unhealthy one. Maybe a woman who had no breast milk for her baby was sold watered-down milk. A vendor could have used weighted scales. There are so many possibilities we could not name them all.
David wrote that while people lie for gain, the Lord sees them and He will “protect the oppressed”. He contrasted the Lord’s promises with those who lie. Verses 6-7 (NLT).