There's a third choice: you report the profit as a capital gain, since you are selling personal assets.
But, there appears to be no gain to report (" I have sold all of these items at a loss") . You are not allowed to claim a loss on the sale of personal property. However, a loss on the sale of "collectables" is technically allowed. However, if everything was sold for less than your cost, it's unlikely the presonal items qualify as collectibles for tax purposes.
Since you only have a loss on the sale of personal items, technically, nothing, normally, needs to be reported on your tax return. However, since you will be getting a 1099-K, you should report it, to head off an IRS inquiry. For how to do that, see: https://ttlc.intuit.com/community/taxes/discussion/i-sold-some-personal-items-on-ebay-and-received-a...
You do not submit cost basis receipts with your tax return. That only comes later, if you are "audited".
If you sold any individual items for more than your cost basis, you are required to report the profit on those item(s) as income. You do not get to reduce the profit on those items by the losses on the other items. Report the sale as "investment" sale, so that the gain is taxed at long term capital gains rates.
