«أُولَٰئِكَ الَّذِينَ اشْتَرَوُا الضَّلَالَةَ بِالْهُدَىٰ فَمَا رَبِحَتْ تِجَارَتُهُمْ»
They chose error in exchange for guidance, and their trade brought no profit.
Choosing misguidance over guidance is represented by buying and selling. The subsequent terms “trade” and “profit” belong to the same borrowed commercial image, giving an instance of tarshīḥ.
Reinforcement extends the metaphor through vocabulary suited to its source field. It heightens the effect without claiming that a literal commercial transaction occurred.