The IRS states that determining the value of donated property would be a simple matter if you could rely only on fixed formulas, rules, or methods. Usually it is not that simple. Using such formulas, etc., seldom results in an acceptable determination of FMV. There is no single formula that always applies when determining the value of property. This is not to say that a valuation is only guesswork. You must consider all the facts and circumstances connected with the property, such as its desirability, use, and scarcity.
Since FMV means the value of the sale if you had sold it to a customer, if you have documentation that can substantiate your valuing of the donated items, you can use the FMV that you feel is most appropriate. Usually, the safest way to determine the FMV is to stay conservative and use the lowest FMV but you could also use an average of various fair market valuations if there are various outlets for the sale of the inventory. Just remember to keep a record of how you determined this FMV in case the IRS inquiries about this donation.
For more information from the IRS about Charitable donations and valuing your donations, please refer to the following links:
Here is an additional link to TurboTax ItsDeductible which you may also find helpful: