Walk through Amos, verse by verse, with the great Bible Commentaries of Matthew Henry (1662 - 1714) & Adam Clarke (1760 - 1832)
{The Lord saith these things, On three great trespasses of Israel, and on four, I shall not convert him, for that that he sold a just man for silver, and a poor man for shoes.} Wycliffe's Bible
2:6
Thus saith the LORD; For three transgressions of Israel, and for four, I will not turn away the punishment thereof; because they sold the righteous for silver, and the poor for a pair of shoes; KJV
Their judges were mercenary and corrupt. They took bribes to condemn the righteous; and even for articles of clothing, such as a pair of shoes, they condemned the poor man, and delivered him into the hands of his adversary.
For three transgressions of Israel, and for four, I will not turn away the punishment thereof; if all these nations must be punished for their iniquities, shall Israel go unpunished? Observe here what their sins were, for which God would reckon with them. 1. Perverting justice. This was the sin of those who were entrusted with the administration of justice, the judges and magistrates, and all parties concerned. They made nothing of selling a righteous man, and his righteous cause when it came to be tried before them, for a piece of silver; sentence was passed, not according to the merits of the cause, but the bribe always turned the scale, and judgment was set to sale by auction to the highest bidder. They would sell the life and livelihood of a poor man for a pair of shoes, for the least advantage to themselves that could be proposed to them;