Throughout Genesis 37-39, there seems to be a switch bewteen terms: some verses say Joseph was sold into captivity by the Midianites, while others say it was by the Ishmaelites. This is heralded as a contradiction.
But is this allegation in harmony with the text? Note the following points:
- In Gen 25 both Midian and Ishmael are listed as sons of Abraham.
- Gen 37:25-28: As they [Joseph's brothers] sat down to their meal, they looked up and saw a caravan of Ishmaelites coming from Gilead. Their camels were loaded with spices, balm, and myrrh, and they were on their way to take them down to Egypt.  Judah said to his brothers, "What will we gain if we kill our brother and cover up his blood?  Come, let us sell him to the Ishmaelites and not lay our hands on him; after all, he is our brother, our own flesh and blood." His brothers agreed.  So when the Midianite merchants came by, his brothers pulled Joseph up out of the cistern and sold him for twenty shekels of silver to the Ishmaelites , who took him to Egypt.
- Note 37:36: "Meanwhile, the Midianites sold Joseph ...."
- Note 39:1: "...the Ishmaelites who had taken him [Joseph] there..."
- See also Judges 8:22 and 24, where in verse 22 the Israelites tell Gibeon that he has saved them from the "hand of Midian", and verse 24 refers to the Midianites as "Ishmaelites". Gideon speaks to the Midianites, and 8:24 says: "And Gideon said unto them, I would desire a request of you, that ye would give me every man the earrings of his prey. (For they had golden earrings, because they were Ishmaelites.)"
It is immediately seen that the author of Genesis (as well as the author of the account of the Judges) is using the terms "Midianites" and "Ishmaelites" in a synonymous fashion. This is obvious to a reader of the text who is not attempting to manufacture a discrepancy.
And even without this internal evidence (which should by itself be convincing in my opinion), the argument for contradiction in these passages hinges on the hidden premise that groups can have but one name and one name only to them. Yet this was not true then, and it is not true now. Even throwing aside the internal evidence presented, what evidence can be adduced for the claim that a group cannot have any synonymous names or identifications?
A general point should now be brought up. Note the internal evidence presented earlier. Now those Skeptics who assert an error here in the autographa are in essence stating that (a) The Hebrew mind is quite unflexible and literarily devoid of synonymoms, and hence can use one and only one name; (b) That the Hebrew author of Genesis is so forgetful that he blows the names only a few verses apart; and (c) That the Hebrew people as a whole are so uncritical and gullible that this "error" went unnoticed as the "error" that it was for a very long period of time.
Can any proof be adduced for any of (a)-(c)? Without any such proof or evidence, points (a)-(c) are really nothing but prejudiced remarks. Write a scholarly paper with the word "Blacks" (say) or "Latinos" (say), substituted for the word "Hebrew" in (a)-(c) and see what response you get. It is really quite easier to let the internal evidence and idiom speak for itself than to believe the necessary assumptions (quite unreasonable in and of themselves) for the Skeptic's claim.
The evidence of Judges 8:24 is especially telling. The condition of being an Ishmaelites is associated with the wearing of a golden earring. This suggests that "Ishmaelites" was not a racial connotation, but a material one. It may be suggested that because Ishmael was a wanderer in the desert, the name was attached to those who were vagrant wanderers with no home base, as might attach themselves to trade caravans for protection. (One will notably not find "Ishmaelites" used in the Bible anywhere else except Ps. 83:6, which offers no discerning information other than that the Ishmaelites lived in tents.)