Simply put, an arabber is someone who sells wares (almost exclusively produce) from horse drawn carts.
The term is specific to the Mid-Atlantic region of the United States where relatively small groups still practice the historic tradition. The largest remaining populations of arabbers are in Baltimore, Maryland and Philadelphia, Pennsylvania (though it is my understanding that the Philly arabbers are now gone). Despite the services they provide to neighborhoods that are otherwise largely without access to fresh produce as well as their historical and cultural significance, they are dwindling in number.
I'll let individuals more expert than I explain the reasons.
The character and situations depicted in the MURDERLAND comic book that reference arabbers are not intended to be representative of arabbers anywhere. All references are intended only to reflect the presence of the practice in the City of Baltimore and at best to expose an unfamiliar audience to a thread in the city's unique fabric worthy of preservation.
Please consult the LINKS section in the upper right of this page for reliable information on arabbers and their trade.