Thank you for your tax question tonight.
County tax valuation on inventory is based on the value of the inventory as of January 1.
Usually, sellers estimate the value of the full inventory.
If you are challenging the county's appraisal, you likely will need to value each piece of inventory (if they are furniture or large items), or find a way to group value small items, in comparison to recent sales of similar items. Otherwise, you will need a non-related appraisal of the inventory, which may cost thousands of dollars or more than you could potentially save in tax, depending on how large of an inventory you have.
Using just cost as a basis for inventory of this type doesn't really make sense, especially if you have held it for decades. Of course, there is a business argument for wondering why you are in the retail business holding inventory for so long. At some point, you would want to mark it down and turn your inventory to cash. So since you have not sold it for 25 years, you could easily argue that the value isn't worth what you paid for it since you can't even sell it! (Unless, of course, you have just ONE antique in inventory and if you sell it you'd have nothing left to sell and be out of business!)
Each year, about this time, YOU have to render an estimate to the county. The county just sends you the rendering forms with last year's appraisal. You are under no obligation to accept the county's numbers. So, for 2016, you will render the value of the inventory on 1/1/16. Only if the county challenges your number do you go to the protest process. Unlike real estate, the county has no idea what you have until you tell them. So if your appraisal was too high last year, you were the one that was supposed to tell the county what it was worth. If you have since changed your mind, you have to file a protest and show that it isn't what you said it was!
So do yourself a favor and render an appropriate value for 2016.