Jack operates a plumbing business as a sole proprietorship on the cash method. Besides providing plumbing services, Jack also sells plumbing supplies to homeowners and other plumbers. The sales of plumbing supplies constitute less than $20,000 per year, and this is such a small portion of Jack's income that he does not keep physical inventories for the supplies. Describe the conditions in which Jack must account for sales and purchases of plumbing supplies on the accrual method.