The attack took place on Thursday, but the news took several days to come to light because Boko Haram has destroyed mobile phone masts in the area.
It was the second major attack in two days by Boko Haram.
In Thursday's attack, the traders were on their way to Chad to buy fish when militants blocked their path near the village of Doron Baga, some 180km (112 miles) north of Maiduguri in Borno state.
Abubakar Gamandi, the head of the fish sellers' association, said the militants had used no guns.
Will Ross in Lagos: "There is supposed to be a multi-national force there but it has not been very effective"
"The attackers killed their victims silently without the use of the gun to avoid attracting attention from the multi-national troops," he told AFP news agency.
Troops from Nigeria, Chad and Niger have been deployed to the area and have a base at Doron Baga, but the military task force has had little impact, says the BBC's Will Ross in Lagos.
On Wednesday, Boko Haram gunmen attacked the village of Azaya Kura, also in Borno state, killing at least 45 people.
In this attack too, victims' hands were tied behind their backs and their throats were slit. The attack was apparently aimed at punishing the community after four insurgents were pointed out to soldiers and were shot dead.
Boko Haram has been waging an insurgency in Nigeria since 2009.
It has stepped up attacks against civilian targets since the Nigerian military launched an offensive against the group last year.
source:punch