"1) How do we calculate the cost of goods sold when only one of the cards inside the package is sold?"
If the package contained 10 cards and you paid $1 for the package, then the cost of that card is 10 cents.
"Under the Tax Cuts and Jobs Act (TCJA), P.L. 115-97, does this mean that we do not need to keep inventory?"
TCJA may or may not have anything to do with your specific and explicit situation. Here's a scenario where I would claim the cost of the cards as a supply expense and just deduct what I paid for those cards, in the tax year I paid for them.
I purchased a box of 100 cards (give or take the exact number of cards) for $100. In the process of working through the box I end up throwing out 30 cards because they're damaged or just plain have no resale value. Of the 70 cards left, I price them for resale anywhere from $5 to $100. During the year I sell 15 cards over the course of the year for a total of $250.
With this scenario I would report the $100 I paid for the cards as an expense, not as inventory. That basically means I get to deduct the entire $100 I paid, in the tax year I paid for those cards.
Then for the cards I sold I'd report as follows in COGS.
BOY Inventory value - $0 (No value here because I already deducted what I paid as an expense)
COGS - $250 (of the 7 or 8 cards I sold, I sold them for a total take of $250 during the tax year.)
EOY Inventory value - $0 (No value here because I already deducted what I paid as an expense)
Now does this better fit your situation? If so, then you're fine reporting what you paid for the cards as an expense and just deducting that cost in full, in the year you purchased the cards. This way you don't have to keep an exact count of cards you purchased, or cards you sold. You only need to track the money, and that's it. Besides, "the money" is all the IRS is interested in anyway.
