Media playback is unsupported on your device Media caption The port of Cotonou is the main pillar of Benin's economy

Cotonou's bustling port is the lifeblood of the Beninese economy. Bags of cocoa and rice, as well as cotton, make up a substantial amount of the port's traffic.

However, alongside these commodities, another precious cargo is being offloaded - used vehicles from Europe and the United States.

"Cars represent a very big portion of the activities of the harbour," says Clement Godonou, operations manager for shipping company Grimaldi in Benin.

"Between 2010 and now, imports grew from 200,000 vehicles to 314,000 vehicles imported last year. It is an important growth in less than four years."

Image caption Many of the second-hand cars in West Africa arrive via Benin

Official figures are difficult to come by, but it is estimated that on average, 25,000 cars are brought into Benin on a monthly basis.

But they don't all stay. An estimated 80-90% are destined for the Nigerian market.

Benin is a major thoroughfare for second-hand cars to West Africa, and in particular to its giant neighbour.

However, the legality of this growing trade is somewhat murky. In 2013, it sparked a diplomatic row between the two West African states after a Nigerian customs official directly accused the Beninese government of complicity in the smuggling of vehicles to Nigeria.

High duties

At the Nigerian border, less than half an hour away from Cotonou harbour, dozens of cars line the side of the road, all awaiting their turn to cross over.

Image caption The used car market in Benin employs thousands of people, at all levels

Image caption Most cars are officially marked for transit to Niger

Flashy stickers adorn the windscreens of the majority of the vehicles marking them as in transit to Niger, though few actually are destined for there, according to Stephen Golub, professor of economics at Swarthmore College.

"Large numbers of used cars are smuggled into Nigeria from Benin and Togo," he wrote in a 2012 report. Those in transit in Benin are "ostensibly mostly to Niger, but in practice, overwhelmingly to Nigeria".

The World Bank's senior economist in Benin, Boulel Toure, says: "The Beninese legally import the vehicles from Europe, but then export them illegally to Nigeria.

"The reason for the rise in this illegal trade is the high import duties which Nigeria imposes on the importation of second-hand vehicles."

Moreover, Nigeria has banned the import of vehicles older than 10 years.

'Good business'

While questionable, the trade in used vehicles forms the base of an extensive business chain in Benin: shipping companies, importers, resellers and intermediaries for Nigerian businessmen all profit from the trade.

The cars in transit are parked not far from the port, waiting for new owners. Dozens of brokers crowd around the entrance to a car market containing thousands of vehicles where they compete for clients.

Image caption Lebanese market owner Abdallah Hachim is confident that trade will continue to be good

Abdallah Hachim built the Zone-al-Madina market in Cotonou, which lies 21 miles south west of the capital Porto Novo, more than a decade ago.

"It is a good business," he says. "If Nigerians keep coming here, it's because it's cheaper. I can assure you that the buyer can save up to 30% buying cars in Benin, rather than buying it directly in Nigeria."

Perusing the vehicles in the market is Adeleke Adekule, a Nigerian car trader.

"In Africa, if you talk about the auto business, Benin is the biggest. The people who come [to] buy cars here, 85% of them are Nigerians and it will continue to grow, because there are still many Nigerians who don't have cars and they want to have their own cars," he says.

Image caption Adeleke Adekule owns a small market in Benin

Armed robberies

The trade has also previously led to security concerns in Benin.

The route from Cotonou to the more northerly border post was infamous for armed robberies. The situation became so problematic that the Beninese customs authority now provides an armed military escort to the border almost every day during the week.

While business is currently good for the thousands of people involved in all levels of this market, the road ahead appears to be deteriorating.

An initiative amongst the Economic Community of West African States (Ecowas), of which both Nigeria and Benin are members, plans to implement a single import tariff across the economic bloc next year.

If this goes ahead, this would see the Beninese car trade lose its competitive edge.