Buy the truth, and sell it not; also wisdom, and instruction, and understanding.
You should buy four things today, and no store carries them. They are truth, wisdom, instruction, and understanding. Do you want them? What will you pay for them?
There is only one way to buy something - pay for it. So buying the truth requires paying a price for it. All buying requires ranking your desires, for no man can afford everything. You spend your limited resources by the priorities you set. And here Solomon tells you to put truth, wisdom, instruction, and understanding at the top of the list (Pr 4:5,7; 16:16).
And once you buy truth, do not sell it to anyone - keep it. Others will offer to buy it from you. How? By offering you pleasure or distraction in exchange for truth. Solomon tells you not to give these four back to anyone for any amount (Pr 3:18; 4:13).
What is the cost? Truth will cost you friends, for most prefer lies, superstition, and tradition (Pr 13:20). Wisdom will cost you ease, for only fools believe and do anything they wish. Instruction will cost you pride, for you must accept correction from others. And understanding will cost you peace, for you will know the world's insanity (Eccl 1:18).
Are there other costs? Yes, pleasure steals time for learning, so give up some of it. Riches steal time, for you must work extra in your vain effort to get them. Give up the foolish notion of being rich (Pr 3:13-14; 8:10,19; 16:16). These four things come by dedication and diligent labor (Pr 2:1-9; 18:1), so you will have to pay the price of effort and time.
If you divide these words by their slight differences in meaning, you might be able to see the following definitions. If you do not, you can see Solomon heaping up good things.
Truth: right, honest, and real knowledge about any subject.
Wisdom: prudent ability to discern and to know what is right.
Instruction: directions and teaching from others wiser than you.
Understanding: comprehending the value and use of truth and wisdom.
Proverbs was inspired to sell these four things, and one of the prices for buying them is to read it often. A chapter of Proverbs a day fits the calendar and will teach you much. So read and apply it daily (action of buying), and do not stop the habit (action of not selling).
Our Lord Jesus bought wisdom (Luke 2:52) until He cornered the market (Col 2:3)! Dear reader, follow His holy example, and He will bless you supernaturally in the doing.