As a Canadian seller, I'm supposed to only charge Canadian customers tax and do not charge or collect any American taxes. However, as has been mentioned, "use" tax on most purchases for some states has come into effect where no matter what you buy, you will/should be paying taxes on it.
It does make sense when you consider buying/selling pre-internet was typically done physically within a region close to you and taxes could be collected rather easily. Now, with so much interstate/international commerce, a lot of governments have been losing out on sales tax proceeds. Can't say I enjoy it though
