This advertisement appeared in The Morning Star [newspaper]. The date of the advertisement is Friday 1 May 1789.
(Carey)
This is a link to a debate about where Equiano was born, in his book Equiano wrote that he was born in Africa but when he was a young slave and again when he was a free man working aboard ships he wrote that South Carolina was his place of birth: http://www.brycchancarey.com/equiano/nativity.htm