You need a tax id if you want to be an adult and sell your work.
this is registering to pay sales tax.
Most street fair/art show/craft fairs whatever require you have them.
I got mine so long ago the memory is blurry.
It involved getting a paper notarized and paying $30 or so to a guy
in one of those little shops on the first floor of an office building
in downtown Brooklyn where they sell packaged snacks, chewing gum, soda and probably cigarettes.
Here's how they do it now
How to Register For A TAX ID (Regular Certificate of Authority)
These links will guide you through the process:
TAX ID/Regular Certificate of Authority
You must apply for a regular Certificate of Authority if you will be making taxable sales from your home, a shop, a store, a cart, a stand, or any other facility from which you regularly conduct your business. It does not matter whether you own or rent the facility.