When the leader of the Boko Haram extremist group threatened to sell hundreds of kidnapped Nigerian girls “in the market” in a rambling online video posted this week, he was not necessarily making an irrational boast.

Doing just that is entirely possible in parts of Nigeria and elsewhere in the developing world, human rights investigators and researchers of child trafficking, sexual slavery and forced marriage said. However egregious it may sound, in some areas the buying and selling of women and children, particularly young girls, has long been an underlying problem.

“It is very well documented,” said Benjamin N. Lawrance, a scholar at the Rochester Institute of Technology who has spent much of his academic career studying and writing about human trafficking. In Africa in particular, he said Tuesday in a telephone interview, “there has never been a period of time where child slavery didn’t take place.”

Professor Lawrance said that if he were to visit any number of West African countries, for example, “I would have no difficulty, within a matter of hours, in finding a place to procure children.”