Some of this may depend on whether you fill out a Schedule C for selling your baseball cards as a "business" or whether it would be classified as a short term capital gain for collectibles.
Other things you would need would be paypal fees, and most importantly Cost of Goods Sold (COGS). That is, you need verified receipts, invoices, etc, stating how much you purchased the items that you sold. (if you purchased on ebay, you may be able to use ebay purchase history.) Good luck!
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