Numerous explanations have been suggested to explain the sequence of events in 37:25-36, but the simplest explanation is the Rashbam’s explanation (on 37:28) that the Midyanites took Yosef out of the pit without the brother’s knowledge and the Midyanites sold Yosef to the Yishmaelite traders, see the elaboration on the Rashbam's explanation by a Shmuel Chayyim son of David Lali quoted by Luzzatto in his comments on 37:28 and by N. Leibowitz, 1976, pp. 406,407. The Rashbam explains that the brothers were eating a little bit away from the pit, which means that they would not have seen the Midyanites come and take Yosef. A possible reason for eating away from the pit was because then they would not hear Yosef pleading for mercy, see 42:21. (Maybe, it was this pleading that alerted the Midyanites to Yosef’s presence in the pit.) In addition, the Rashbam explains that when Yosef said to the brothers that they sold him, this should be understood to mean that you (the brothers) caused me (Yosef) to be sold.
According to this explanation the brothers did not sell Yosef, and there are two proofs for this approach. One, 37:29,30 records that Reuven returned to the pit and was surprised that Yosef was not in the pit. Yet, if the brothers sold Yosef, why was Reuven surprised when Yosef was not in the pit? The answer based on the Rashbam's approach is that Reuven was surprised since the brothers did not sell Yosef, and then he could not understand what happened to Yosef. Two, after Yosef had incarcerated his brothers in Egypt, they talked freely amongst themselves, and they expressed remorse for not heeding to Yosef's pleas, 42:21. Why did they not express remorse for selling Yosef? The implication is that they did not sell Yosef, as argued by the Rashbam.
The Rashbam's approach has raised several questions. One, how could the brothers not have known that the Midyanites took Yosef out of the pit? Even if they were not sitting in view of the pit, still they would not have heard the noise and commotion of the Midyanites? Two, why were the brothers other than Reuven not surprised that Yosef was not in the pit? Three, if the brothers did not sell Yosef, why did the Torah record Yehuda’s proposal to sell him? Four, when Yosef said to his brothers that you sold me, can one really understand that Yosef is to be understood to mean that you caused me to be sold?
These questions on the Rashbam's explanation can be answered by realizing that the brothers attempted to actualize their plan to sell Yosef immediately after Yehuda proposed it. 37:27 does not record the brothers acting upon Yehuda's proposal, but they had to act quickly since the Yishmaelites were travelling and if they waited, then they would have missed their opportunity. This need for speed to talk to the Yishmaelites meant that they did not take the time to remove Yosef from the pit, but went immediately to the Yishmaelites to work out a deal.
When all the brothers left to speak to the Yishmaelites, the pit was not being watched, which means that nobody would have seen or heard when the Midyanites pulled Yosef out the pit. As the Rashbam notes, the word, va-yaavru, signals that they happened to be passing by and by chance they passed the pit where Yosef. They then picked up Yosef and met up with the Yishmaelites who were talking to the brothers. The brothers then saw Yosef and he saw them. He started to plead with them, 42:21, but they did not save him and the Midyanites sold him to the Yishmaelites.
When the brothers were negotiating with the Yishmaelites, Reuven realized that this was his chance to save Yosef, so he went back to the pit where Yosef was, 37:29, but during the time when brothers left the area near the pit to speak to the Yishmaelites until the time that Reuven returned, the Midyanites had already pulled Yosef out of the pit, 37:28. By the time that Reuven returned to his brothers, 37:30, Yosef was gone with the Yishmaelites traders.
The understanding answers all the questions on the Rashbam's approach. The brothers were not near the pit when the Midyanites took Yosef out of the pit (first question) since they had gone to make a deal with the Yishmaelites traders. The brothers were not surprised that Yosef was not in the pit (the second question) because they knew that the Midyanites had taken him out of the pit since they saw Yosef when he was sold by the Midyanites to the Yishmaelites traders. It is possible that they never told Reuven that they had seen Yosef being sold so he thought that Yosef had been killed, 42:22. The discussion between the brothers to sell Yosef (the third question) is significant to the story since the brothers acted upon this suggestion, which enabled the Midyanites to take Yosef out of the pit. In addition, when Yosef saw them when he was sold by the Midyanites to the Yishmaelites, he considered them to be accessories of the sale because from his point of view they were cooperating with the Midyanites by not stopping them from selling him to the Yishmaelite traders. Thus, (the fourth question) he stated that they sold him, 45:4,5, and he did not mean that you caused me to be sold. On the other hand, the brothers never thought that they sold him, and instead they believed that their sin was that they had not heeded Yosef's pleas to stop the Midyanites from selling him to the Yishmaelites, 42:21. In this case, both Yosef and the brothers viewed the same situation from different perspectives.
With regard to the Medanites, while many claim that they are Midyanites, this cannot be. 25:2 records the children of Avraham and Ketura, and in this list, there is Midyan and Medan, which implies that they are different people. Also, 37:36 refers to the Medanites selling Yosef to Potifar in Egypt, while the Midyanites sold Yosef to the Yishmaelites in Israel. What is the role of the Medanites in the sale of Yosef?
The answer is that Medanites were the brokers who arraigned the sale for the Yishmaelites. 37:25 records that the Yishmaelite traders were spice merchants. Why was this information mentioned? The answer is that it teaches us that they were spice traders and not slave traders. They bought Yosef from the Midyanites and brought him to Egypt, but in Egypt they needed help to sell Yosef. The Medanites were the brokers who arranged the sale of Yosef from the Yishmaelite traders to Potifar. Thus, 39:1 records that Potifar bought Yosef from the hands of the Yishmaelite traders, and this signaled the change in ownership of Yosef. However, 37:36 records that Medanites sold Yosef to Potifar, since when brokers arrange for a sale, they say “I sold X,” meaning they were the cause of the sale but not that they were the actual sellers.
In conclusion, Yosef was sold twice, once by the Midyanites to the Yishmaelites, and then from the Yishmaelites to Potifar. With regard to the first transaction, the brothers were aware of the sale but were not actively involved in the sale. With regard to the second transaction, the Medanites were the brokers to the sale but they never owned Yosef.
Leibowitz, Nehama (1905-1997), 1976, Studies in Bereshit, translated by Aryeh Newman, Jerusalem: The World Zionist Organization