The heart of the believer should never be sold to the world and Satan. In Proverbs 4:23 we see that we must put effort into keeping our heart as we seek the Lord and His ways.
Are your beliefs for sale? As the world tries to sway us this way and that, Christ wants us to be solid in our beliefs and values and follow Him. Proverbs 23:23 makes it clear that we need to stand upon the truth that can only be found in God.