What is the ruling on brokerage, and when does it become permissible or forbidden?

Originally, brokerage (simsara) is permissible, based on the Almighty's saying: "And for he who brings it is [the reward of] a camel's load, and I am responsible for it" [Yusuf: 72]. Also, based on Al-Bukhari's statement: "Chapter on the broker's fee," and Ibn Sirin, Ata, Ibrahim, and Al-Hasan saw no harm in the broker's fee. And based on Ibn Abbas's saying: "There is no harm in saying: 'Sell this garment, and whatever exceeds such and such is yours.'" And Ibn Sirin's saying: "If he says: 'Sell it for such and such, and whatever profit there is is yours, or between me and you,' then there is no harm in it." The permissibility of brokerage is contingent upon it not leading to something unlawful, such as usury (riba), or selling or dealing in something whose sale is forbidden. Otherwise, it becomes unlawful, due to the cooperation in sin and aggression involved, which is forbidden by Allah Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty" [Al-Ma'idah: 2].