Keep track of the cost of the items you sell to track or prove your taxable income. The best way is to create a spreadsheet or some other record for each sale listing the sale price and cost.
You can always deduct the cost of the items you sold. Other deductions depend on whether you are selling items as a business to make a profit or as a hobby.
Getting rid of junk sounds like hobby income, meaning you can only deduct cost of goods sold AND you cannot claim a loss, so if you sold an item for $100 that you bought to $200, then your deduction against your Form 1099-K would be limited to $100.
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