>If I buy a vintage computer second hand for $1500 and then manage to sell it to someone else for $2000, I don’t owe taxes on that.

Uh, in the United States? Yeah, you absolutely do [0, 1]:

>"If you make a profit through these activities, it’s considered taxable income. You can use the Form 1099-K, along with other records, to determine how much tax you owe."

>"Remember that all income, no matter the amount, is taxable unless the law says otherwise – even if you don’t get a Form 1099-K."

>"If you made a profit or gain on the sale of a personal item, your profit is taxable. The profit is the difference between the amount you received for selling the item and the amount you originally paid for the item."

You may wish to review your understanding and confidence in your understanding of tax law.

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0: https://www.irs.gov/newsroom/are-you-making-extra-cash-selli...

1: https://www.irs.gov/businesses/what-to-do-with-form-1099-k