A bit of a warning before you read the positives and negatives the positives are more for the British than the slaves as they had no positives about the horrible way they were forced to work.
Positives
The British got more cotton being produced.
Negatives
The slaves were working in horrible conditions
They didn't get paid
They got sold off to the wealthy to work for them
The slaves got kidnapped and sometimes rounded up by the main tribe and so didn't see it coming