Is it permissible to sell appliances in installments, taking an initial down payment and securing them through an intermediary who receives a percentage of the profit, without the buyers knowing the intermediary's percentage or the source of the appliances?

If your friend sells appliances in installments, he must not sell anything until he owns and possesses it, as if he buys the appliance from you for himself or shares with you in the ownership of the commodity. However, if people request appliances from him out of friendship, then he is an agent for them, and it is not permissible for him to profit without their knowledge. He must introduce them to the seller, and it is permissible for him to agree with them on a fee for the agency and to take a commission from you for brokering.