‘And the Midianites, merchantmen, passed by and they drew and lifted up Joseph out of the cistern and sold Joseph to the Ishmaelites for twenty pieces of silver. And they brought Joseph to Egypt.'
The ‘they' is the brothers. They had seen the caravan in the distance and now it approaches to pass them by. So they draw Joseph from the cistern and sell him for twenty pieces of silver, the price of a man. As we have seen above Ishmaelites were known as Midianites, but distinguished by their gold earrings from other Midianites.
“Twenty pieces of silver.” The price of Joseph as a slave at 20 shekels of silver is correct for that period. We know from external sources that in the late 3rd millennium BC the price of a slave was 10-15 shekels, but by 1800-1700 BC it was 20 shekels. In 15th century Nuzu and Ugarit it was 30 shekels (compare Exodus 21:32) and by the 8th century BC it was up to 50-60 shekels (2 Kings 15:20). This is remarkable confirmation of the accuracy of the narrative.