Each state may have different rules. Merchants typically will need a "seller's permit".

For California merchants, see Board of Equalization for who requires a seller permit and how to obtain one.

The article "Do you need a Seller's Permit" states, "When you hold a seller’s permit, you must; file sales and use tax returns, and pay any sales or use tax due on your sales and purchases. You must report and pay sales tax on each taxable sale. At the time you make the sale, you may collect from your customer an amount equal to the tax you will owe."

To learn more about your rights and obligations of owning a California's seller permit, see Publication 71

To learn more about Sale and Use Tax.