This is general education, not legal or tax advice. Rules vary by state and country; talk to an accountant for your situation.
If you start selling without forming anything, you're typically a sole proprietor. It's free and simple: your business income goes on your personal tax return. The downside is that there's no legal separation between you and the business.
An LLC can add a layer of liability separation between your personal assets and business debts and claims. It usually costs a formation fee and recurring state fees, and requires keeping business finances separate. It doesn't automatically reduce your income tax.
For the sales-tax and income-tax basics, see sales tax and dropshipping.
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