I'm always amazed at buyers in the US that are confused and shocked that there would be Duty on an item they are buying from or was manufactured in, a foreign country. Like, you have never traveled outside the US and had to fill out a Customs Declaration on your way home or going across the border?!
I thought it would be obvious...
In any event, I have a copy of the US Harmonized Tariff schedule, the 8500 series pages, that I copied off of the Internet. That covers all the elctronic stuff we're concerned-with in Audio. I can see what the Duties are, if any, and I tell the buyer at the beginning when doing a shipping quote. If you call FedEx or UPS and ask them what the Brokerage fees will be (usually based on the value of the item that is being shipped), they will tell you - another mystery cleared-up.
I have shipped overseas many times and have never had a problem. I have never had a buyer request a zero value, and I wouldn't do it anyway, as that is ridiculous and bound to attract attention at Customs. We work out a realistic value that will allow it to slide through Customs and the buyer has to understand that the package cannot be insured for a value higher than the declared value. So if it gets lost or damaged, they are assuming that risk.
I also get them to pay by wire transfer - no PayPal unless they have a ton of good feedback, and no Money Orders.