Some background on Roman slavery

I got side-tracked into looking up Roman slavery: apparently its extent is not well attested and consequently estimates are disputed. I hadn't realized that exposed babies might provide a slave pool--I'd heard of exposed girls being raised to be slave prostitutes, so it makes sense that boys might be raised and sold too. Some might have been bought from captors outside the Roman Empire--the surrounding nations fought each other too. "Adverse moral judgments focus on the supposed greed and general turpitude of slave dealers, who were accused of tricking out their wares to defraud buyers, and likened to pimps"--note that the concern is for the customers rather than the slaves.

His estimate of the free population in the cities and countryside is 6.5-7.5M and 49-52M respectively, while estimates for slave populations are 1.3-1.9M and 3.5-6.5M respectively: 20%±2 slaves in the city vs 10%±3 in the country. They weren't all captives--children born to a slave woman (provided she was a slave throughout the pregnancy) were also slaves.

But what's known? There's no data from Republican Rome so: "We are reduced to the mere assumption that slave prices in Republican Italy ought to have been relatively low during the massive expansion of the regional slave complex."

The relative prevalence of private sales versus transactions arranged by professional dealers is unknown. In slave markets, slaves were displayed on platforms and could be undressed for closer inspection; new arrivals were marked with chalked feet. Slaves wore placards (tituli) advertising their qualities around their necks (including their origin, state of health, and propensity to run away), or special caps (pillei) in those cases where the seller would not offer guarantees. Extant sales contracts, primarily from Egypt with rare additions from Italy and Dacia, testify to the scrupulous observance of formal legal requirements, and give us a rough idea of the age distribution of traded slaves, dominated by individuals in their teens and twenties.

You'll probably have read already about diffences between Roman slavery and ante-bellum US slavery (and Caribbean slavery, different yet again). This is interesting background detail--and explanation of what's not known.