You've already received good answers, so I can't add much. But I'll try.
If you are generating much more than a grand in income you probably ought to start reporting it because you might get noticed. If you're reporting it, you need to keep receipts of your expenses as legitimate offsets. Sold a knife for $200, but got $50 in materials? You pay income tax on $150. But you have to keep the receipts and add them up at the end of the year and file them away in case you're ever audited.
I go to H&R block to get my taxes done for a couple hundred bucks. Hell, the expense is also tax deductable.
But the real reason to start reporting is because you're allowed to report all legitimate expenses as tax deductions. Want a new $1,000 grinder. It is a tax write off, so really it only costs you $750ish. Power, internet, even gas can be legitimate write offs.
Talk to an accountant first, it isn't very expensive. You'll possibly want to register a business name, possibly open a business bank account, and perhaps even register for a federal tax ID.