An inventory is necessary to clearly show income when the production, purchase, or sale of merchandise is an income-producing factor. If you must account for an inventory in your business, you must use an accrual method of accounting for your purchases and sales.
To figure taxable income, you must value your inventory at the beginning and end of each tax year. To determine the value, you need a method for identifying the items in your inventory and a method for valuing these items.
The rules for valuing inventory cannot be the same for all kinds of businesses. The method you use must conform to generally accepted accounting principles for similar businesses and must clearly reflect income. Your inventory practices must be consistent from year to year.
The value of your inventory is a major factor in figuring your taxable income. The method you use to value the inventory is very important. Generally there are two methods for valuing inventory. These methods are cost or lower of cost or market.
Source: IRS.gov