Correct, you are being paid by a marketplace, but again it is not a "gig", so you do not need to read further into the rules regarding gigs. If it turned out you made a profit on these personal items that you were just trying to clear out, not make a part-time job out of, that income would go on Schedule D, where investments are reported.
Next year, if you receive a 1099-K, you will need to report it. But if the items are sold at a loss again, there is a box for you to check in TurboTax to indicate that the items were personal items sold at a loss, and there will be no tax computed or Schedule C.
