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How is income tax calculated in relation to selling used items?
Yes. This income would be reported on schedule SE. Normally, you will not owe any tax if the amount is less than $400.  Practically, $100 in a garage sale is not why the IRS created the form SE. I wouldn't lose sleep over keeping track of small cash sales over the course of a year. However, if you have the information I'm not going to tell you not to report it.