This word:﴾أُوْلَئِكَ﴿ “Those” refers to the hypocrites who say,
﴾آمَنَّا بِاللَّهِ وَبِالْيَوْمِ الآخِرِ وَمَا هُمْ بِمُؤْمِنِينَ﴿“‘We believe in Allah and the Last Day,’ but they are not believers.”
The opposite of the word Shara’, meaning ‘to buy’, in business terms, is Bai’ ‘to sell’. The letter ‘ب’ is affixed to the thing which is left (or sold), and in this case, guidance.
These people have taken misguidance and left guidance. This is a business transaction which is non-profitable as they have used guidance as a payment in their transaction.
A question might arise here: What guidance did they possess that they could give in exchange for misguidance?