Thanks for clarifying, I'm expecting to get a 1099-K next year so I played around with TurboTax to investigate. Some basic observations that I hope will be helpful:
If you exceed the sales threshold (which is cumulative from all selling platforms like Reverb, Ebay, Paypal, etc) you'll get form 1099-K which means your sales have been reported to the IRS.
The 1099-K will list your gross sales, but this does not mean you will owe taxes on this amount. It is up to you to determine how much of this gross amount was actual gain. You compare your proceeds (the amount you received for the sale of the item) to the cost basis (the amount you originally paid for the item) to determine a gain vs a loss.
As mentioned, a loss on the sale of an item cannot be deducted from your income unless you are a business, but it will offset all income received from that sale and result in zero gain for that individual sale.
If you profit from the sale of an item (proceeds exceed the cost basis) then you owe taxes on the difference. In other words, you only owe taxes on the amount of money that was profit.
You cannot accumulate losses from the sale of individual items to offset the gains made from the sale of other items. So for example, lets say you sell 6 items in a given year. 5 items resulted in a loss of $100 each, and 1 item resulted in a gain of $500. On paper, you have lost $500 and gained $500, but the IRS assesses each sale individually so the losses simply zero out and you owe taxes on the $500 gain.
TurboTax makes it quite easy to input your sales and calculates everything for you, then spits out the forms you need to submit with your tax return. To me, this all becomes fairly inconsequential, as 95% of my sales I either lose money or break even. So the main penalties that I face are selling fees from Reverb or G&S fees from Paypal, but I just increase my asking price 10%-20% on Reverb and that's still where I make most of my sales. Yes you have to deal with the risk of buyer fraud and returns but luckily those are few and far between.