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.