What exactly, was Yehudah's reason for changing the original plan and selling Yosef to the Yishme'elim?
Seforno: Not only would they gain nothing by killing Yosef, but they would lose, inasmuch as their consciences would give them no rest. Moreover, killing him would not even serve as a warning for those who would harm them, since they would be obligated to keep his death secret 3 to prevent their father from finding out. Selling him to the Yishme'elim as a slave, on the other hand, would serve as a fitting punishment for his having intended to rule over them
Ramban: Reuven taught the brothers that it was better to throw Yosef into the pit and let him die (thereby causing his death) that to kill him with their hands. Yehudah now added that causing his death too, would be akin to murder. 4
Hadar Zekenim: Since we cannot kill him openly, this is not revenge.
Hadar Zekenim - "v'Chisinu Es Damo" - if we sell him, his Damim (value) will go in our Kis (pocket). Rosh - right after, it does not say 'rather, let us sell...', rather, "let us sell." This implies that also "v'Chisinu Es Damo" refers to selling.
Over and above the fact that Yosef was their brother, and it would be wrong to sentence him to death (Refer to 37:26:1:3).
This is also how Rashi and Targum Onkelos interpret "and we will cover his blood". Whereas the Ramban explains it literally, anyone who commits a murder that he wants to hide, buries the body and covers the blood, as we find in Shmos, 2:12).
למה קרא יהודה להשלכת יוסף לבור "נהרוג את אחינו" והרי יתכן שיוסף לא ימות שם?
גור אריה (פסוק כב): גם אם הוא לא ימות מהנחשים והעקרבים, הוא ימות שם מרעב.