Inventory involves both the P&L and the Balance Sheet. Although purchases only in NOT an inventory change, it does mean that you are changing the Balance Sheet and possibly the accounting method.
Here is why - In order for you to utilize just the purchase of merchandise in your Cost of Goods Sold (CoGS), you will have to ensure ALL the inventory reported at the end of the previous year was completely sold and there is no inventory remaining at the end of the current year. This will mean the Balance Sheet Account for Inventory is $0.
Therefore, going forward you will report only the purchases. This will indicate you are changing to a CASH only method of accounting, for the most part, unless you still utilize invoicing for customers with 30-60 days to pay, which would allow you to still use accrual basis.
If inventory is not at $0 at year end, you will either need to write-off the inventory you currently have on hand, (at the price you paid for it, which will reduce your Net Income), or wait until you sell ALL the current inventory at a mark-down price (thus minimizing the write-off). Either way, you will need to have the Inventory on the Balance Sheet until it is sold and the revenue or write off is on the P&L.
If you are considering changing your Inventory method of reporting altogehter, however, from the LIFO method, this is far more complicated and may require the assistance of a tax practitioner for guidance.
In either situation, chagning from ACCRUAL to CASH Accounting or the LIFO method of Inventory, you would be required to file a Tax Form 3115. More information on this tax form may be found in IRS instructions here - https://www.irs.gov/instructions/i3115#en_US_202212_publink100035819
