Here`s a good article I found on this very subject -
How to Determine Your Sales Tax Charges
Apparently it also comes down to where your merchant account provider is located - so you would have to charge for some states, but not others.
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Kari Rippetoe
Marketing Manager
goWholesale
http://www.gowholesale.com
kari.rippetoe@gowholesale.com