yes, there is a sales tax function in eCart, and in Power Store.
You are responsible fro collecting sales tax for any items that ship to the same state as your stores physical location (in the case of a brick and mortar store) or the state where the store is registered.
So if you are an online store only, and registered in California, you are responsible fro collecting taxes for any purchases that are shipping to an address in California.
For orders that are shipped out of state, it is the customers responsibility to report those purchases and pay taxes when they file their taxes at the end of the year.
you're store will pay the taxes to the IRS in the same way, when taxes are filed at the end of the year.
These are really legal questions that you should be bringing up with a tax accountant though.