It's describing an arbitrage opportunity. If person A wants to buy something for $5.00, and person B will only sell for $1.00, I stand to make $4.00 per unit b

It's describing an arbitrage opportunity. If person A wants to buy something for $5.00, and person B will only sell for $1.00, I stand to make $4.00 per unit by simply being a middleman for the transaction. Private or public markets, the arbitrage opportunity remains the same.