I am not a tax adviser and this is not tax advice. Please consult your CPA
With that said, generally all businesses need to know the value of their inventory at year end to provide to their tax preparer.
This is a simple thing to do in Seller Central. You basically just run a report.
This needs to be done late December 31 or early Jan. 1 ( so basically log into Seller Central and run the report before you head out to your New Years Eve party)
You don’t have to do anything immediately with the report, you just need to run it at that time to show the value of the inventory you have at years end.
Here is a video on how to run the report:
The basic steps are:
Go to “Reports”, then select “Fulfillment.”.
Click “Amazon Fulfilled Inventory”, then click “Request Download”
After it is complete, download it to your computer.
It is a .txt file that is hard to read. You should open it up in Excel ( or Open Office or Google Drive ) and rename the file and save it in a safe place.
Here is a really good detailed post on what to do: