Sorry, but stealing someone's land and turning them into a slave is not a free market. No one is putting Rome up as an ideal for its economic or political freedom. But why would the wealthy people even bother stealing the land in the first place? Because once they were able to control that capital (the land) they could produce more grain than subsistence levels and turn a profit. This incentive allowed Rome to produce so much grain they could offer a public dole in spite of its other ethical problems.