In the USA this depends on if you wish to open a merchant account.
Most if not all merchants require a business certificate.
If you wish to use services such Paypal or 2checkout then you don't need a certificate.
The cheapest way is to do a Doing Business As ( DBA ) which can be filed in your local Court House. A small Corp is better but costs more to setup.
Really this depends on how serious you are about your business.
If you are just starting out and have a small monthly income I would just use your SSN and use PayPal and a CC service like 2Checkout.
This way you can see if this business is really what you want to do.
This however limits you is some ways in that billing becomes a small problem using these services. But you can get around these minor problems.
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