I get this question way too much. Here’s my answer: A Tax ID is a nine-digit number issued by the IRS to businesses (and other entities) for tax reporting. Has a similar function to the Social Security Number. In fact, the form you use to request one is the SS4. That’s one thing. What you typically need to give to a vendor, is issued by your state and is called different things. In California, it’s called a Seller’s Permit, some states call it a resale permit or whatever. But it’s issued by your state. I wish vendors would stop using these two terms interchangeably as they are very different things.
Whether or not the vendor requires a tax-whatever-they-call-it is not your concern. Your concern is whether or not your state requires you to have one. In California, if you have more than two garage sales per year, or sell at the flea market more than twice a year, you are required to obtain a Seller’s Permit and remit sales tax for your sales. Some people think that because they are a small business, they don’t need to get one, but they do. So check with your state’s Department of Whatever to see if you need one.
Some states don’t require them. But, if you get one and set yourself up properly, you won’t go around worrying about whether or not a vendor is going to ask for a copy.