I am not a tax accountant. If you are purchasing items for resale and doing it on a regular basis, this would be considered a business which would require Schedule C for income and expenses and Schedule SE for self employment taxes. You would be required to pay around 15% self employment taxes on the net income and pay federal income tax at your marginal tax rate.
According to Poshmark's web site, the 20% is a sales commission you are paying to Poshmark for what they do. It is not for taxes. If someone is just occasionally selling items of their own clothing for less than what they paid for it, it would not be a business and it would not be reported on their tax return.
