"and he chooses not to keep track of inventory."

A Consignment operation means there is no Purchased inventory. They sell first, then pay the consignor the sales amount minus a commission for selling. Your client has not purchased anything at the time it is put on hand and made available to sell.

Inventory is not "non-incidental materials and supplies." While you might be thinking that "stuff = stuff" there are different types of Stuff. Here is an example of material and supplies: I run a water district and buy a few meters and a spare pump/motor to keep on hand at all times. I "restock" as they get used. They are not on hand for sale. They are a supply item.

"would increase the Cost of Goods Sold (COGS)"

Well, it cannot do Both. It cannot be COGS (a direct expense of what got sold) as well as Expense as Supply (an indirect expense). Supplies and materials might be used in the course of making goods or providing services. In this example, if I buy various wood products to make furniture to sell, that furniture has the invested cost of goods still right there, on my shelf, ready to sell. I also might have a bunch of leftover scrap materials that I consider can be useful, but I want to write off all of the material that was purchased this year, even the part not used to make furniture, yet.