Taxpayers with 1099Ks from selling collectibles on eBay and related sites is reported as on the sales on Schedule D, Capital Gains and Losses. They have to select the item is "a collectible" checkbox for proper reporting.
Selling collectibles is not a business and should not be reported Schedule C. Reporting them to a Schedule C requires taxpayers to have an inventory and track that.
Regularly selling items which are not collectibles on eBay may be considered a business by the IRS.