The taxes are like a sales or use tax. And that is a tax that local governments want to impose every chance they get. It's just like when you buy a used car. Every time that car gets sold to a new owner, the state wants to collect another sales tax on the transaction. In NY State, they actually expect you to pay a use tax to NY (if the state doesn't get to collect sales tax) on any freakin purchase you make. So, if you purchase a bass from San Diego over the Internet, and no sales tax is collected, they expect you to declare the purchase and pay the use tax!!
Duty is another thing. Duty is not due on any product (or at least virtually any product) that originates in, and is sold between, two NAFTA countries. The way way to avoid imposition of duty is to provide a Certificate or Origin, which is actually pretty easy to do. The form can be downloaded by the purchaser, and filled out and signed by the seller. If anyone needs a link for the form let me know.