«أُولَئِكَ الَّذِينَ اشْتَرَوُا الضَّلَالَةَ بِالْهُدَى فَمَا رَبِحَتْ تِجَارَتُهُمْ»
These are the ones who purchased misguidance in exchange for guidance, so their trade yielded no profit.
Choosing misguidance over guidance is depicted through purchasing. The later words profit and trade belong to that same commercial field and strengthen the initial borrowed picture.
The reinforcement lies not simply in the buying expression itself but in adding associated details appropriate to the source image of commerce.