Earlier, they saw a camel-train of Yishma'elim approaching. Who were the Midyani merchants?
Rashi: This was a different group of people. And it teaches us that Yosef was sold a number of times
The brothers made a point of communicating with the Yishma'elim (Refer to 37:25:2:1) rather than with the Midyanim, who were likely to appear in Chevron to sell their merchandise. Rosh - the Midyanim saw the rich Yishma'elim from afar, and bought him when he was still in the pit.
R. Bechayei: Yosef told his brothers "You sold me to Egypt" (45:4), i.e. I was sold through your throwing me into the pit. Rashbam - alternatively, the brothers commanded the Midyanim to pull him out of the pit before selling him to the Yishma'elim. See Oznayim la'Torah, who derives from the Pasuk in Amos, 2:6 that it was the brothers who initially sold Yosef and not the Midyani merchants.
Riva, citing R. Tam: The brothers were afraid to take Yosef from the pit, due to the snakes. The Yishma'elim hired the Midyanim to take him out through witchcraft; we find that the Midyanim who went to Bil'am were witches. Since they took him out, it is as if they sold him. (If they saw the snakes, they should have realized that Yosef survived because he is a Tzadik! Perhaps they thought that he is a snake-charmer, like Riva said above (22)
What are the implications of the words "Va'yimshechu Va'ya'alu"?
She'arim d'R. Hai Ga'on: It implies that Kinyan Meshichah acquires only after one has moved the object away from its original location.
What is the significance of the twenty silver pieces?
These are Kodesh Shekalim which are synonymous with Sela'im. Hadar Zekenim. One Shekel (Sela) equals four Dinarim, so five Shekalim are 20 Dinarim.
Yerushalmi (Shekalim, 2:3): And it is because each of the ten brothers received half a Shekel (Binyamin was not present at the sale) that Yisrael had to give half a Shekel as a Kaparah. See also Ba'al ha'Turim. Mar'eh ha'Panim (2:3)
Someone as beautiful as Yosef is surely worth more than 20 silver pieces!
Moshav Zekenim, Hadar Zekenim from Pirkei d'R. Eliezer: Due to fear in the pit 1 and of the snakes, his appearance changed, and he was sold for only 20 silver pieces.
Bechor Shor: Due to their hatred they were very eager to sell him, and sold him for far less than his real value.
Hadar Zekenim: They sold him for 20 silver pieces for each brother, i.e. 200 in all, like the Gematriya of "v'Evyon Ba'avur Na'alayim" (Amos 2:6).
Rosh (32) #2: The 20 pieces were Zekukim (pure silver - which were worth much more than regular Dinarim).
Rosh (32) #1: His appearance changed, due to moisture in the pit. Also, fear inhibits blood (also this changed his appearance).
Why did the brothers consent to sell Yosef?
Ohr ha'Chayim: Their anger was calmed after they lowered him so much - they knocked him down, stripped him and shamed him, and cast him in the pit. Primarily, they wanted to ensure that his dreams not be fulfilled. Being sold permanently for a slave precluded becoming free, and all the more so greatness!