Who sold Yosef, and who knew?

Question

I had always understood that Yosef's brothers sold him to the Ishmaelites. We know that they planned to do so, but during this week's torah reading I noticed this for the first time:

כח וַיַּעַבְרוּ אֲנָשִׁים מִדְיָנִים סֹחֲרִים, וַיִּמְשְׁכוּ וַיַּעֲלוּ אֶת-יוֹסֵף מִן-הַבּוֹר, וַיִּמְכְּרוּ אֶת-יוֹסֵף לַיִּשְׁמְעֵאלִים, בְּעֶשְׂרִים כָּסֶף; וַיָּבִיאוּ אֶת-יוֹסֵף, מִצְרָיְמָה. 28
And there passed by Midianites, merchantmen; and they drew and lifted up Joseph out of the pit, and sold Joseph to the Ishmaelites for twenty shekels of silver. And they brought Joseph into Egypt. (Gen 37:26)

The "they" is ambiguous. It could mean that the brothers had the Midianites do the deed for them -- earlier Reuven says "let not our hand be on him"; the implication is "let's not kill him" but maybe it's more general? Or it could be that the Midianites came along while the brothers were eating and not paying attention, and their plans to do the deed were thrwarted? Rashi's comment on this is just to say that this shows that Yosef was sold several times, which doesn't seem very helpful.

So my questions are:

1. Did the Midianites act on their own (and collect the money) and the bothers weren't part of the transaction? Or did they act as agents for the brothers?

2. If the Midianites acted on their own, what did the brothers know? Did they know he'd been sold or only that he was gone from the pit? (Were they possibly as surprised as Yaakov 22 years later upon learning that Yosef was alive?)

Answer 1

Read this Shi'ur by Menachem Leibtag. http://www.tanach.org/breishit/vayesh/vayeshs1.htm

Here's a teaser for it:

"After throwing your brother into a pit to die, would you be able to sit down to eat? Yosef's brothers did, as the Torah tells us! However, the Torah does not tell us if they sat near the pit, listening to Yosef's screaming and pleading, or if they sat far away from the pit, to enjoy some peace and quiet. So what difference does it make? Believe it or not, this tiny detail affects our understanding of almost every aspect of the story that ensues. In this week's shiur, as we study the story of Yosef and his brothers, we will entertain each of these two possibilities -showing how this 'missing detail' leads several commentators to conclude that the brothers may never have sold Yosef after all!"

Bottom line, Rashi and Radak hold it was the brothers (probably because Yosef accuses them when they are reunited), but others (including Rashba"m and Hizkuni) recognize that this may not be (indeed probably is not) the case. See Menachem's novel explanation regarding Yosef's accusation.

Answer 2

See summary on TorahMusings.com. Partially quote below.

...Ramban and Sforno hold that the Ishmaelites and Midianites were working together. The brothers sold Yosef to them, and they sold him to Potiphar in Egypt. Ibn Ezra says that the Ishmaelites and Midianites were the same people...

Rashbam, [et.al] say that the brothers didn’t sell Yosef. Rather, the Midianites took him out of the pit, sold him to the Ishmaelites, who sold him to Potiphar in Egypt.