So, you could consider it a hobby or just sales of personal property? Was this a "one-time" event or will you continue to buy and sell baseball cards as a hobby or business?
If you consider it a hobby, then you would include the income on the other income line of your return.
However, IRS would consider this the sale of collectibles and the sale of collectibles is considered a capital gain/loss situation.
Please refer to the following link for additional information:
Sale of collectible baseball cards
