> But they are still buying it and still taking the risk that it won't sell.

I don't know about Costco, but the big box stores don't carry any risk. If the product doesn't sell then they return it and deduct that off your next invoice. The same happens for a customer return.

Depending on the retailer/product category you might get charged slotting fees, pay-to-stay, plus a share of promotional costs[1]. There are also other fees for use of distribution centers, discounts for paying early (even if it's not early), fees (or extended terms) for slow-moving products, and so on.

"Owning the items" feel less correct than "borrowing items with a promise to pay".

[1] https://traxretail.com/blog/quick-guide-shelf-space-costs/