What's the ballpark threshold for having to report cash you got to the IRS? Proceeds from a garage sale seem like they would be way too low to bother reporting, but I don't know at what point not reporting it would be illegal.
All income requires reporting, and cash payments received over $10k (lump sum or multiple payments for a single transaction) must be reported to the IRS on Form 8300.
Would it not be illegal at literally any point? If you make one dollar or a thousand, the IRS wants to know. Maybe you meant at what point they would investigate, but that's dependent on too many factors for me (not an American) for me to even guess at.
Perhaps the easiest way to gauge it is "If I earned this much in tips as a server, would I likely report it?"
Rule of thumb (for countries subject to FATCA) is $10k. After that, the tax department (of the local government) starts to look into the source of income.
