Form 1099-K reports payments received through third-party payment processors like PayPal, Venmo, Square, and online marketplaces. It's issued when you receive over $5,000 in business transactions in 2024 (thresholds are changing — check the current year's rules). Selling personal items at a loss doesn't create taxable income even if you get a 1099-K; selling business goods or services does. The IRS has been phasing in lower reporting thresholds — eventually $600 — which will result in many more people receiving this form. Keep records distinguishing personal sales from business income.