You can sell your used goods, the stuff in your house that you no longer use. However, if you are purchasing items for purposes of selling at a profit, you are in a business and self-employed. And the amount of your monthly profit and the hours it takes you to earn that amount would need to be reported to SSA. It may or may not be considered Substantial Gainful Activity. Probably not enough to matter, but some people totally support themselves by buying and selling items. Profit matters.