Get your taxes done using TurboTax

There is a category for collectibles and since these are collectible cards, that seems the best way to report them. 

First, report it in the Investments section where you can also enter the cost of the collectibles. 

Then enter the Form 1099-K as it is received, in the Other Common Income section, so that it matches what was reported to the IRS.  
TurboTax will prompt you to link the two entries.

For TurboTax Online:

1. Go to Wages and Income
2. Scroll to Investments and Savings, select Show More
3. Select Start for Stocks, Cryptocurrency, Mutual Funds, Bonds, Other (1099-B).
4. On Your Investments and Savings Summary screen, you’ll see a line item for your 1099-K payment. Select Edit or Review and enter the information for your personal items sale. 
   • Be sure to select Collectibles under the What type of investment did you sell?
   • The Cost or other basis amount should be the amount you originally paid for the item.
5. Select Continue when you’re finished to go to the Review your Sales screen.
   • If there was more than one collectible item sale included in the 1099-K amount, select Add another sale. You can group together sales but should not mix sales with losses and sales with gains, or sales for items held less than a year with sales held more than a year as these gains are taxed at different rates.
      

Next, return to the Wages and Income main screen and scroll down to Other Common Income to enter the Form 1099-K.

Note:  This screen mentions a Form 1099-B but you do not need one to report the sales. 

Or, various dates

Since you don't have the actual costs that you paid for the cards you'll have to come up with a good-faith estimate for what you paid for them.  That will require figuring out what the amount that you paid for an average card was and maybe for some specific cards.  Just keep all records of what you do in order to come up with the estimate in order to have on hand in case you are asked about it.

@colby_parker