Hello @DrFrank and thanks for your question. First off to disclose, I am NOT a tax professional, but here is my understanding and approach of your situation as someone who owns/runs a retail shop. If you are selling items to the public, then your state sales tax rate would apply. Example, for my state my sales tax for items like books is 6 percent. Square is setup with my state tax rate, so every sale adds in this tax rate. I remit this amount to my State Department of Revenue that collects Sales Tax payments. Check with your particular state tax agency or own accountant for your sales tax liability that you are responsible for. You probably would have to apply for a Sales Tax License to be able to collect and remit this money.

Your husband and other authors wouldn't be responsible for the sales tax, as you are the one selling the books to the public. The authors in a sense are "selling" the book to you (basically the cost of Square's processing fee for the transaction). The authors technically are receiving income through this sale, so each one would be responsible on their own income taxes to claim this as income.

I hope this helps answer your question, and again please remember I am not an accountant or tax professional, so when in doubt please consult a professional advisor.