You do need to report the income from the sales, even though you will often have a loss. These losses can help to offset your other income. You can also deduct the costs associated with the sale of the items such as your flea market fees, payment processing fees, miles driven to and from the flea market to sell, supplies used (pens, receipt books, price stickers, etc.).
When you sell the item, your "basis" in the item is the income that you recognized when you received the item and wrote your review for it (the "ETV" from the itemized report on your Vine Account page). As you've noted, when you sell something for $50 your "cost" is the $100 of income you recognized giving you a $50 loss. When you sell $0 ETV items, you will have a profit.
The easiest way to record this is to add all of the totals in sales to your "other income" for the business in TurboTax and then list the recognized income of the items sold as either cost of goods sold or an "other" expense noted as something that makes sense to you. I use this second method and the description "recognized income" so that it's clear to me what the expense is, should I be invited at a later date to show some sort of support for this number to the IRS.
When reported as a business (either separate from your Vine review writing or combined with it), these are deductible losses.
You could report the sale of these items on the same Schedule C as the income from your Vine 1099-NEC or on a separate Schedule C. Use whichever method is easier for you.
Sales of the items, in some cases, would be more appropriately reported as the sale of personal items. Sales of personal items can't create a loss that offsets other income, where sale of the items as part of a business can.
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