Any time a book or artwork is sold in Massachusetts, sales tax is owed. If your item is selling in a store, the store takes care of that. You simply get your share of the profits. The same is true with Amazon. But if you are at a library, or a festival, or somewhere else that a buyer is handing you cash, you are responsible for the sales tax for that transaction.
There is no minimum amount. Massachusetts wants their share no matter what.
You get your Vendor ID for FREE online. Technically you are required to have that certificate with you at any event where you are selling. Some fairs require it to even give you a table. It’s good to print out a copy of the certificate and carry it with you and your supplies.
You then file your results once a year, also online. You generally do that around tax-time, which is when you’re figuring out all that financial stuff out. In Massachusetts it’s 6.25% of direct sales. So you just figure out how much you took in for your item sales, multiply by 6.25% in person, and pay that. It’s good to keep track of exactly how many books / art items you sell at each event, and the total income, just for general record-keeping reasons. I keep a file folder of them for each year.
Here are the links and details. I wrote this up for artists selling paintings, but it’s the exact same theory for authors.
Ask with any questions! If you’re confused, just start by getting a Massachusetts Vendor ID. Just focus on that one step. Let me know what help you need while working on that.