1) I believe that it sets the tone for his account, describes his attitude toward the book and gives an overall impression of Equiano himself. It shows his work is not meant merely for entertainment but for the purpose of promoting the inhumanity and torments of slavery.
2) a. What Equiano is saying is that the slave traders were so greedy that they didn't think about the long term - just about the short term. In this case, what that means is that they packed too many slaves into the ships because they wanted to make as much money as possible. Because they did this, the living conditions for the slaves were terrible and many of them became sick. This was improvident