I’m not sure “efficiency” is the right way to think about it. The realistic counterfactual was probably not the people who were slaves instead being immigrants with rights to their own person, but rather staying in Africa
Now perhaps it’s true that “not having the labor at all” is worse than “having the labor but in inefficient slavery”, but that doesn’t really mean much in terms of comparing it to modern day market systems, where we have the labor and (relatively more) free markets.
Or to go back to the middleman issue of slave owners.
It’s beneficial for A and B to engage in mutual trade of goods and services. If A and B would not have traded unless C was involved, and C takes a 20% cut yet A and B still want to trade despite that, then C is probably an economic good. Introducing buyers to sellers (and vice versa) is a whole job in modern economies after all.
However if A and B were already going to trade, then C coming in and parasitizing 20% by threatening violence against the two participants is not an economic good. Introducing a violent parasite into a free trade hurts the value generated in trade. This is not exclusive to thieves or slave owners, it happens even with normal taxes. Now perhaps the thief, slave owner or government uses the money in some other manner to promote something society cares about more than economic efficiency, but it is not in question that the violent middleman is a drag on the economy.
Violent middlemen are parasites. Would one a priori think that your trip to the grocery store would be cheaper if on the way a highwayman held you up and charged you a “toll”. No. They don’t provide any value themselves, they extract from you. The same thing with slave owners, what value they might create in the role of manager or buyer/selling matching is incidental to the main point of highwayman style parasitism, and is beaten out by proper managers and matchers.
And this isn’t even a modern idea. Adam Smith talked about the inefficiency of slavery himself.
Lest we forget, Smith openly condemned slavery on both moral and economic grounds. As an economist, he argued that slavery was inefficient and ineffective for society. He believed that when people are forced to work, and therefore cannot act upon their “own interest”, they have no incentive to innovate, improve or invest their skills and labour.
Ok yeah my bad, economic good is a term that already has a specific meaning and I meant it more as that C is a good thing for the overall economy lol. I mean it more like this:
A: Has a Pikachu card but wants a Charizard card.
B: Has a Charizard card but wants a Pikachu card.
Normally A and B would just trade each other, but what if they don’t know the other exists or can’t find each other? In comes our “matcher” C. Maybe A values the Charizard card as (Pikachu + $2) so he pays C a dollar to find a person willing to trade, they find B, a trade occurs and everyone’s life is better for it. It is a good thing that C did the job of matching A and B together.
Matchmakers, brokers, marketplaces, etc are examples of matchers. A grocery store has an actual economic purpose that creates value instead of extraction.