Where you purchase the equipment and then sell it to the buyer, I would treat that as inventory.
In a case where you just arrange the purchase and the transaction is between your client and the equipment provider and you never touch the money as it passed from the buyer to the seller, you have nothing to report.
As you stated, in the case where the equipment provider gives you a commission, you already know how to handle that.
In my limited experience, this is basically what a roofer does. (My brother is a roofer). When he quotes a price for a new roof, it includes the cost of the materials. He purchases the materials and records it as inventory. Then when he completes the new roof he reports the sale of that inventory as "a part of" the quoted price for the new roof.
In a case where the client provides the materials he has nothing to report in the cost of goods sold section.
