 Benjamin became a specialist in commercial law , of which there was a great deal in New Orleans ' busy river port — a center of international commerce and the domestic slave trade . By 1840 , the city had become the fourth largest in the United States and among the wealthiest . Many of the best lawyers in the country practiced commercial law there , and Benjamin successfully competed with them . In one case , he successfully represented the seller of a slave against allegations that the seller knew the slave had incurable tuberculosis . Although Benjamin tried some jury cases , he preferred bench trials in commercial cases and was an expert at appeals . 
