If you don't regularly sell items on Facebook Marketplace, you don't need to report this as Business Income.
For a one-time income, you could report it as Less Common Income > Miscellaneous Income 1099-A, 1099-C > Other Reportable Income under the Wages & Income section.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"