As an economist...that's not what trade meant at all during the time Austen was writing, or in general. When writers referred to the slave trade, they called it such (Adam Smith does this in The Wealth of Nations, but even earlier, in the Salamanca School, which represents the earliest roots of modern economics, there's no such tradition of trade = slave trade). If actual economists aren't writing about 'trade' = 'slave trade' (especially given the breadth of topics within trade and more generally industry, I highly doubt it was used that way in layman's terms.