I pay Retail to MLM company for inventory/purchases. They credit me back the commission % and add it to a 1099 at end of month. Some of that inventory is never sold due to personal use, used for samples, incentive gifts, etc.
See if the following applies to your situation
I believe that even if you might not need to keep "inventories"; you would have to account for the "inventory" as supplies and deduct the cost of the supplies either when sold (or disposed of), or when paid for....whichever is later. Thus, I don't think that you can pay for the supplies in 2015 and deduct the cost if the supplies weren't sold until 2016, etc.
But read the publication and the references in the publication carefully.
See also Pub 535 for general business expenses