Sales Tax in the United States is not a Federal responsibility but rather, a state, county, and municipality one, established by what is called, having a nexus.  

Nexus simply means that you have an established relationship and presence in a specific state. If you have a formal relationship (nexus) between your business and a state, then you probably need to charge sales taxes there.

Remember, nexus can be physical or economic. Even if you are a remote seller. Each state has its own guidelines, and nexus is determined using sales from the previous or current calendar year, based on the amount of revenue from sales to customers.  

Make sure you constantly review your total sales to see if you are now tax liable!