There are two possible state issues.
1. Do you have a self-employment business? You are required to collect sales tax according to the laws of your state, and pay them according to some schedule and some forms. You will have to look at that in more detail, the volunteers here don't really help with collecting sales tax.
2. On many state income tax returns, the state will have a line to pay sales and use tax. The concept is that if you buy items out of state (and don't pay sales tax to your state) or if you buy items online (and don't pay sales tax at all), your state can legally charge use tax. (It's the same as sales tax except that instead of paying at the point of sale, you pay after you bring the item into your state and start using it.) It has always been the law that consumers who bring products into their state without paying sales tax must pay use tax to the state, but almost no one ever did it. So states started asking residents to self-report their out of state purchases on their tax return and add the sales tax to their income tax.
Some states ask you to estimate the amount of stuff you bought out of state. Some states allow you to use an estimated amount that the state provides that looks at your income (spending money) and makes some assumptions about average spending. Or, you can say "No, I did not buy anything on which I did not pay sales tax" and not add the amount to your income tax return.
If your state is assessing you use tax, they may think that you buy products out of state and have failed to pay sales tax on the purchase, and have failed to voluntarily pay use tax.
You need to look at the tax forms and instructions for your state, and your state laws, to see what you can do. You may need to reply with a letter showing that you don't make untaxed purchases (for example, because amazon always charges sales tax). But if you buy a lot from places that dont' charge sales tax (like eBay) you might have a problem.
You need to figure out what they are telling you and then think of records and arguments you have that will prove them wrong.
