How is sales tax determined?

If your US-based business has a physical presence in a state, such as a store, office or warehouse, you must collect applicable state and local sales tax from your customers. If you do not have a presence in a particular state, you are not required to collect sales taxes.

The physical presence is known as a "nexus." Each state defines nexus differently, but all agree that if you have a store or office of some sort, a nexus exists. If you are uncertain whether or not your business qualifies as a physical presence, contact your state's revenue agency. If you do not have a physical presence in a state, you are not required to collect sales taxes from customers in that state.

Here’s A Guide to Sales Tax Nexus in All U.S. States
http://blog.taxjar.com/sales-tax-nexus-guide/

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