USA Inventories; cash or accrual basis

[JohnHarris]

According to IRS Pub 538:

The following taxpayers can use the cash method of accounting even if they produce, purchase, or sell merchandise. These taxpayers can also account for inventoriable items as materials and supplies that are not incidental (discussed later).

A qualifying taxpayer under Revenue Procedure 2001-10 in Internal Revenue Bulletin 2001-2.

A qualifying small business taxpayer under Revenue Procedure 2002-28 in Internal Revenue Bulletin 2002-18.​

Assume you do meet the requirements. Can you write off (expense) purchases made for inventory in the year you bought it. If not, why not?

Thanks