What does Paul mean when he says that a person is sold under sin or the curse?
Paul means every child of Adam is born sold under the curse and under old Adam, sharing Adam's sinful offspring rather than Christ's sinless, divine blood. This describes the natural condition from Adam. For the believer, old Adam is reckoned crucified and dead in God's mind, though his influence remains in Christian life.
Les explains that every child of Adam is sold under the curse and is under that old Adam. This is contrasted with Christ: the blood circulatory system of a fetus originates with the father and is not touched by the mother's blood, so in the virgin birth Mary could carry Jesus without her blood coursing through His veins. His blood system originated with the Father, so it was divine and sinless, unlike the offspring of Adam. Les then notes that Paul writes as the believer who has reckoned old Adam crucified and dead. When a person believes the Gospel, God saw the believer in the person of Christ on the cross. Yet old Adam remains an influence even in Christian life. Les maintains that old Adam is crucified and declared dead in the mind of God, which is possible because God is omnipotent and omniscient, with whom nothing is impossible.