Remember that as a hobby, your sales are capital transaction, long term capital gains if held more than 1 year and short term capital gains if held less than one year. However, comic books are considered "collectibles" and as such, are taxed at a higher rate than other capital gains. You need to list each item you sell on form 8949 with the purchase date, purchase price, sell date and sell price. With personal items, capital losses are not deductible and do not offset capital gains, so if you sold 100 books with a total loss of $1000 and you sold one book with a gain of $500, you have taxible income of $500.
Going forward, keep a detailed spreadsheet. You know the price of each even if the receipt is not itemized. For past purchase, make your best guess and hope you aren't audited.
