As you fill out those taxes here's a little mood music .
https://www.youtube.com/watch?v=YtksJEj2Keg
https://www.youtube.com/watch?v=YtksJEj2Keg
You are right that in general inventory is not an expense until sold but the way it is accounted for differs depending on whether you are a hobby business or a for profit business and also whether you use the cash or accrual method of reporting. For a hobby business you're totally correct. You need not ever take a physical inventory of anything. You can't write off obsolete, stolen or damaged inventory.In general, blanks that you intended to make a pen from and sell (that pen) are not deductible until the pen is sold. Until a blank is sold whether resold to another party or turned into whatever, it is considered inventory (an asset) and therefore, not deductible.
In general, inventory does not become a business expense until is sold.
If if are selling inventory (or making pens) purchased years ago, you can assign a value to it based on what you think it is worth (based on some external source) or what you paid for it (historical cost) when you purchased it. Usually the safest option is to use what you paid for it. Again, this is all in general.
Not that Dave needs me to validate his advice, but all that he has said agrees with my education, training, and experience. In fact, I don't actively practice tax prep, so I am sure Dave and his wife are better equipped to answer than I am.
What do most people use to record all this in and our $$