Many words lose their relevance and thus usage over time. Fortunately, slavemongering, is among them.
A monger is defined as a dealer or trader. To monger is to promote or deal in something specified. It is generally used in a composition. Thus a costermonger is an itinerant fruit-seller, a fellmonger is a dealer in skins, a fishmonger peddles fish, and an ironmonger is a dealer in iron goods.
Walter William Skeat’s An Etymological Dictionary of the English Language, suggests as possible source of the word.
monger, mangere a dealer merchant formed with suffix ere from mang ian – to traffic barter gain by trading.
The relationship to the Lat[in] mango, a dealer in slaves, is not clear but the [English] word does not appear to have been borrowed from it. 
Slaves were “inseparable from any species of private property” and since “titles to slaves were transferable,” their owners had the power of conveyance.  Thus the legality of slavemongering, or the trafficking of slaves, was firmly established in the Antebellum South.
Slave owners seemed somewhat ambivalent to the idea of slavemongering. Some despised the idea but took advantage of it non-the-less. Thus one who found the sale of a bondsman distasteful or even bordering on amoral, could unburden themselves of their human property using a professional.
Franklin & Armfield had the distinction of being the the largest slave-trading enterprise in the South at least during the period prior to the Panic of 1837. Formed in 1828, its principles were John Armfield and Isaac Franklin. They each “accumulated fortunes in excess of a half million dollars” before retiring. 
Hundreds of small entrepreneurs engaged in slave trade but only a few made large sums of money. Among these few was Nathan Bedford Forrest who “was the largest slave trader in Memphis during the late 1850’s; he was reputed to have made a profit of $96,000 in a single year.” 
 Walter William Skeat, An Etymological Dictionary of the English Language, accessed on the internet via Google Books, November 22, 2008 here.
One of the concepts Millett and Maslowski mention in their book, For the Common Defense: A Military History of the United States of America, is the Fabian Strategy. It refers to an approach by one side in a military conflict who avoids big decisive battles in favor of small engagements designed to wear the opposition down, reducing their will to fight and their numbers by attrition.
The term is attributed to Quintus Fabius Maximus Verrucosus (ca. 280 BC-203 BC), a Roman commander who used the technique in fighting Hannibal during the Punic Wars. He harassed Hannibal’s army through small engagements and cut off their supply lines but avoided getting pulled into a decisive battle. Needless to say, the strategy requires time to succeed. Because of this, it also requires the support of the governing powers on the side that adopts it because there is no decisive showdown event. In Fabius Maximus’ case, the Romans politicians listened to his detractors (peer commanders) and replaced him with men who would confront Hannibal head on. They were resoundingly defeated at the Battle of Cannae (pictured right). The Romans eventually went back to the method of battle avoidance and harassment as designed by Fabius and eventually succeeded in driving Hannibal back to Africa.
The Fabian Strategy was used during the American Revolution by Continental forces against the British. While politically unpopular, Washington agreed to adopt it. Interestingly, the idea for its use came from Nathaniel Greene.
I’d be interested in thoughts from my readers on use of the Fabian Strategy during the American Civil War. While I have yet to study in depth the exploits of Nathan Bedford Forrest (pictured right), my sense is that this kind of harassment of the enemy was a forte of his Tennessee Cavalry. I’ve also heard the phrase “removing the Fabian” associated with Sherman’s march through the south. No doubt this refers to the ferreting out of harassing guerrilla-type forces.