The returns gained by traders from the sale price of slaves were offset by both the fall in the value of land, that resulted from the subsequent decrease in the marginal productivity of land, and the fall in the price of output, which occurred due to the increase in market size as given by westward expansion. Kotlikoff suggested that the net effect of the inter-regional slave trade on the economy of the Old South was negligible, if not negative. Speculators created slave trading companies which operated on both ends of the market, with firms such as Franklin and Armfield, based in Alexandria, Virginia, with offices in Louisiana, enjoying immense profits.