The traders were returning from a rural market in the Riyom district of Plateau state when they were ambushed by "unknown assailants" on Tuesday, said police spokesman Tyopev Terna.
"Eleven people who were returning from a weekly village market in Makera village were shot dead at about 7:30 pm (1830 GMT) yesterday," he told AFP.
Four other traders were injured in the attack, which happened some 30 km south of the state capital, Jos, heh added.
Plateau state lies in Nigeria's so-called Middle Belt that separates the predominantly Muslim north from the largely Christian south.
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It has long been a hotbed of ethnic, sectarian and religious tensions between indigenous Christian farming communities and the Muslim settler Hausa/Fulani cattle herders.
Tensions typically boil over into tit-for-tat violence over access to land and resources, and the struggle for political control.
Nura Abdullahi, head of cattle herders union in Plateau, said: "We don't have an idea who the victims were and whether the attack was ethnically motivated or the work of bandits who abound in the area.
Riyom district has been hit by waves of violence between farmers and herders.
Last month two people were killed in Jol village in the area after a young herder's body was found nearby. The cattle drivers were blamed.
At least 29 were killed in an attack targeting people sheltered in a rural primary school in nearby Bassa district.
The attack was apparently in revenge for the death of six Fulani herders who were killed by unidentified assailants in the area.