Lack of GAAP, apparently. So who sold him?

I think it's usually just oversimplification. The points of the story are Jacob's family, and the ministry of Joseph in Egypt. The incidentals get glossed over in the retelling.

The MacArthur Study Bible, KJV edition, says that they did. In both locations. What version are you using?

I think I see your point. Verse 28 says that the Midianites pulled him out of the pit and sold him.

Too many authors, not enough collaboration. Salaam

The New American standard version leaves out that the brothers pulled Joseph out and sold him. However, the KJV, Amplified, and the NIV all say the brothers pulled him out, and sold him. All of the versions say in vs 27 that they planned to sell him, and later in Gen 45 vs 4 Joseph himself states that "I am your brother Joseph,whom you sold into Egypt." So I would say that in the entirety of the story, and using other versions along with the NASB (which happens to be my favorite do to the complete acuracy of it) I say the fact that his brothers sold him into slavery is true. Great question, and I had some good reading again!

There are many translations and they either say Joseph's brothers sold him or the Midianite Merchants did. I looked at a few translations that point to the midianite merchants. They were: New American Standard, King James Version, English Standard, New World Translation. When I read it, I tend to lean toward that the Midianite Merchants found him in the waterpit unattended and used that opportunity to make some money. Not a nice thing to do but it happened nonetheless. I realize that his brothers did discuss selling him but what makes me think they didn't was the fact that at Genesis 37:29, Reuben returns to the waterpit and is surprised that Joseph is gone and returns to his other brothers and exclaims so. My theory is this: After throwing Joseph in the water pit they first discussed killing him and then changed that plan to selling him. All the while, Joseph heard their deliberations. In the meantime the brothers left Joseph unattended while they went and did whatever, maybe make sure their flocks were ok. Now, here come the Midianite Merchants who find Joseph and perform that transaction. Joseph wouldn't be the wiser because he probably still thought it was his brothers who made the deal and just had the midianites come and get him. That is why in Egypt Joseph exclaimed that "...it is I, Joseph, whom you sold into Egypt..." Joseph may have heard about the plan to sell him but didn't realize the details surrounding it transpired quite differently.

I think you may have misread this. Joseph's brothers did sell him. In Genesis 37:26 it reads, 'Come, let's sell him to the Ishmaelites" and then in 37:28b it reads, "and his brothers pulled Joseph out of the cistern and sold him for twenty shekels of silver." Genesis 45:4 agrees with this where Joseph stated, "I am your brother Joseph, the one you sold into Egypt!" This agrees with the previous scripture. Not sure where you read there they didn't sell him.