If you have a business, you are required to keep accurate records and report your income and expenses, regardless of whether you get tax paperwork. You will generally only get a 1099-K from a payment processor like PayPal or Squarespace. Does FBM actually process payments or is it just an advertising platform?
If you are only selling your own used items for less than your original cost, you don't have taxable income and you probably don't even have a small business. Schedule C is used to report your small business, where a business is an "ongoing trade or business" that you regularly engage in for a profit motive. Selling used items for less than you paid is not profit (even though it is cash to hand) and won't usually be a business unless it you do it "regularly" (more than occasionally selling used items).
If you are buying inventory and reselling it for more than you paid, then you are probably a business. You report your gross income (sales), and you can subtract the cost of items and other "ordinary and necessary" expenses. You pay income tax and self-employment tax on the net profit. You report your income from your own accurate records, regardless of what tax paperwork you get.
