Shloymie: Wait a minute, Rabbi! Didn't Yosef's brothers sell him? Doesn't Rashi says that "vayimsh'chu" means Yosef's brothers?
RRW: Indeed, Rashi does. But read Vayeishev 37:28 thru 36. Where does it specifically show us how Yosef's brothers sold him? We also we notice that Reuven [38:29] was clueless about the sale. This seems a bit strange.
Shloymie: Ok, that might be a bit weird, but doesn't Yosef specifically say that his brother sold him in Parshat Vayigash 45:4 ?
RRW: True, but what about Binyamin?
Shloymie: Just look at the 10 Harugei Malchut! It's obvious that Yosef's brothers sold him!
RRW: Shloymie, you must be agreeing with your namesake Rashi. However, I'm not quite "Rashidox" about this! :-) The simple p'shat is that the Midyanim intercepted Yosef while Reuven was on his way and sold Yosef to the Yishma'eilim.
Shloymie: What about Vayigash and the 10 Harugei Malchut?!
RRW: What about the simple Read, and the Rashbam who gently suggests that this is the P'shat? :-)
RRW: OK, I see WHY Rashi pinned this on the brothers, though Rashi probably based this interpretation on Vayigash, not Vayeishev.
Shloymie: What's wrong with that?
RRW: Nothing, but I can read it simply here and still answer your point on Vayigash.
In Vayigash, Yosef accused his brothers of selling him, because:
a) They had said that they'd tried to do it.
b) That deed was l'maaseh done, even though through Midyanim.
Do we really know who sold Yosef?
Yosef had a right to accuse his brothers of selling him since they'd started the chain of events. This seems to mean that even when they occur through others' actions, aisi HKBH are "mitztareif machshavah ra'ah l'maaseh".
Alternatively, maybe Yosef just thought that his brothers asked the Midyanim to do it, didn't realize that Reuven was clueless and assumed that they just beat the brothers to the "punch."
Maybe we shouldn't take vaymish'chu to mean that the brothers actually SOLD Yosef - only that they caused the sale to happen. I'm not saying that Rashi is wrong, just that we have a viable "davar acheir". I prefer this explanation..