Slave Trade in Washington DC from 1861:  The Civil War Awakening (pages 58-60)

“Few people – at least outside of Washington – noticed that the 1850 law did not actually prohibit slave trading itself. It simply banned anyone from bringing Negros into the District of Columbia for the purpose of selling them out of state.  That took care of those embarrassing coffles (a line of animals or slaves fastened or driven along together.) Washington would no longer be a major entrepot for Negros being shipped off to the slave hungry Cotton Belt from overstocked Chesapeake region. But it was still legal for a Washingtonian to put his house servant up for public auction, and even to advertise the offering, as Green and Williams did in the pages of the Daily National Intelligencer, the city’s leading newspaper and a semi-official chronicle of congressional proceedings. If an unlucky slave happened to turn up the following week in one of the Alexandria  slave pens right across the Potomac, ready to be packed onto a New Orleans bound slave schooner – well that was perfectly within the law.” 

Advertisement for a slave named Willis, the valued property of Honorable Judge George M. Bibb (deceased) one of the most distinguished  longtime residents of DC