In Chapter I of Book IV of The Wealth of Nations, Adam Smith makes the following assertion:
The transportation of commodities, when properly suited to the market, is always attended with a considerable profit; whereas that of gold and silver is scarce ever attended with any.
But he does not back this up with any data or reference. Nonetheless, he continues:
When those metals are sent abroad in order to purchase foreign commodities, the merchant's profit arises, not from the purchase, but from the sale of the returns.
What I understand from this is that the merchant books in the profit once he sells the commodities he bought from abroad. Fine, that makes sense (accounting wise). But why couldn't the return be non-zero? As per Smith's first paragraph quoted, the transport of metals is supposed to yield no profit.