Who purchased Joseph, the Ishmaelites or the Midianites?

Who purchased Joseph,
the Ishmaelites or the Midianites?
Genesis 37:28, 37:36 and 39:1

1. Ishmaelites (Genesis 37:28) - "Then some Midianite traders passed by, so they pulled him up and lifted Joseph out of the pit, and sold him to the Ishmaelites for twenty shekels of silver. Thus they brought Joseph into Egypt ."
2. Midianites (Genesis 37:36) - "Meanwhile, the Midianites sold him in Egypt to Potiphar, Pharaoh’s officer, the captain of the bodyguard ."
3. Ishamaelites (Genesis 39:1) - "Now Joseph had been taken down to Egypt; and Potiphar, an Egyptian officer of Pharaoh, the captain of the bodyguard, bought him from the Ishmaelites, who had taken him down there."

According to Achtemeier, Paul J., Th.D., Harper’s Bible Dictionary, ( San Francisco : Harper and Row, Publishers, Inc.) 1985, the term "Ishamelite" was synomous with the term "Midianites." They were probably references to the same general group known to have decended from Abraham. Ishmael was born to Abrahm through Hagar (Genesis 16), the hand maiden. The Midianites were descendants of Midian, a son of Abraham and his concubine Keturah (Genesis 25:1-2). Additionally, "The term ‘Midianite’ probably identified a confederation of tribes that roamed far beyond this ancestral homeland, a usage that explains the biblical references to Midianites in Sinai, Canaan, the Jordan Valley, Moab, and Transjordan’s eastern desert. (Achtemeier, Paul J., Th.D., Harper’s Bible Dictionary)