Hi Srpieper,
Since you didn't not purchase these items with the intent to profit from reselling them, this wouldn't be business nor hobby income. You most likely paid more for each item when new than you ultimately sold it for in used condition. The cost basis would be the lower of the cost you paid for them or the Fair Market Value when you converted them from personal use to resale. What's the Fair Market Value (FMV)? The going price on marketplaces including eBay / Poshmark could be used as FMV. Unless you sold rare items that increased in value after you purchased them, your cost basis would be the same as the amount the eBay / Poshmark customers paid you. When personal items are resold, gains are taxable and losses are non-deductible.
Here's a link to a similar question that details the steps to take to report the 1099-K amounts as Other Income and an additional entry of negative amount for your costs that offset that income. Be sure this offset amount doesn't exceed the sum of the amounts reported on the 1099-K forms you receive because that would cause your other income to be negative (loss), which isn't allowed for the sale of personal items such as used clothing. The end result is that you've reported this income but it's not taxed because of the offsetting costs. I recommend Option B detailed here: 1099k paypal for personal items
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