The smugglers in Libya work as international shippers: they never travel with half-empty vehicles.
During the outward, from south to north, from the deserted slopes of the Sahel towards the Zuwara and Sabrata coasts, they carry men; when they return to the Niger, Mali and Chad they carry petrol and diesel.
How can be explained this returning traffic?
It is easy to understand if you go to any gas station in Tripoli. The premium gasoline costs, at the pump, 0,150 dinars per liter (price paid on 3/5/2017) that is, at the real exchange rate of the dinar, less than 0.025 euro cents. Just think, for comparison, that a bottle of water can cost 0.50 dinars, that is more than 300% of the gasoline price.
How can be explained this so low price of gasoline, whereas Libya produces oil but doesn't refine it, and therefore the gasoline must be bought back from Italian refineries? Simple: because gasoline is a subsidized good.
The bloodless treasuries of Tripoli government put the difference, in various ways, to keep the more and more impoverished Libyans quiet. This allow various militias and criminal organizations to take advantage of these subsidies to acquire large amounts of refined petrol and diesel, which then they export illegally to the countries of the Sahel, Africa and Egypt, doing a roaring trade.
But this is not all: this kind of traffics has spread to Italy, as we are told by very direct witnesses. First in jerry cans, passing from Tunisia, then in ships, with the change of documents in Malta.
In traffic to Italy this two markets, that of men and the one of fuels, are rejoined. In fact, in both cases, transnational organized crime of Italian origin plays a key role.
Map of Libya depicting oil and gas pipelines, refineries, oil ports, closed oil ports and the main migration routes used by human traffickers.