What effect does the speaker wish to achieve by using the words commodities and barter in his speech?

Horrors of the Slave Trade

speech by William Wilberforce,
delivered May 12, 1789 (excerpt)

We see then, in the nature of things, how easily the practises of Africa are to be accounted for. Her kings are never compelled to war, that we can hear of, by public principles, by national glory, still less by the love of their people. In Europe it is the extension of commerce, the maintenance of national honor, or some great public object, that is ever the motive to war with every mona

Question

What effect does the speaker wish to achieve by using the words commodities and barter in his speech? 


Horrors of the Slave Trade
 
speech by William Wilberforce, 
delivered May 12, 1789 (excerpt)
 
We see then, in the nature of things, how easily the practises of Africa are to be accounted for. Her kings are never compelled to war, that we can hear of, by public principles, by national glory, still less by the love of their people. In Europe it is the extension of commerce, the maintenance of national honor, or some great public object, that is ever the motive to war with every mona