Is it permissible to purchase a portion of the stagnant goods, or to participate in their purchase without contracting in one's personal name, or to receive a commission from the buyer in return for the sale?

Brokerage (mediation) is permissible, and it is permissible for the broker to take a fee (commission) from the seller or the buyer or both.

As for the broker buying the commodity for himself or sharing it with another buyer, there is a difference of opinion among scholars regarding this. The preponderant opinion is that he is not allowed to do so unless the owner of the goods explicitly or customarily permits him, for fear of him favoring himself.