The hobby income designation is confusing because collecting comics/art is a hobby but for the vast majority of the people on this forum selling expensive comics or art you would not file it as hobby income.
Example of hobby income: you spend $5,000/year on photography equipment and supplies but make $100/year income as a photographer. You do not get to deduct $5,000.
Example of selling comics: you sell a comic for $1,000 that you paid $500 for 6 months ago. You put it on Schedule C, sales $1,000, cost of sales $500, cost of shipping supplies $10, shipping $25, net profit you'll pay tax on is $465.