No one is denying that most sellers here would rather ship CONUS only but for international buyers there are a few things you can do to help sway the seller's to your side. I've not had many transactions here, only 2 so far... One from Australia and one from the US. Both went well without any incident. I've also bought a few knives on eBay and from NGK.
First thing you could do is to send them links to your feedback on this or other forums. This is why I always bookmark those I've received. I also try to give them my ID on other forums (in my case, Spyderco forums, KnifeBST etc) and my eBay feedback page; all of which only shows knife purchases.
I also brief them on my local import laws concerning knives which means no autos (i don't even take chances with Kershaw's flipper knives), balisongs and any knives that has both sides sharpened (dirks, daggers, stilettos). Check your local statutes, laws and by-laws. Most countries have these info online or in your public libraries. If it's illegal in your country don't buy it. No point whining if it gets confiscated and no reason to blame the seller for your mistake.
You also need to know what is the maximum amount you can buy before import taxes are levied, you can't place that responsibility on the seller. My limit is SGD400 (approx USD270). Don't ask the seller to commit fraud by underdeclaring the amount, I don't... If you do that's your prerogative. The only thing I ask for is for it to be marked as camping knife, not "tactical black bladed folder with scary serrations".
I do realise it may cost more to ship overseas but there has been times where the cost is not as astronomical as you think. NGK shipped me 2 knives for USD9.98 with delivery confirmation. I try to steer them to the USPS website to check the rate sometime. I don't mind losing a few dollars now and then either... Link them to your local postal service website to show that tracking is available when the knife leaves the US.
If all this fail... Accept it like an adult and move on. It's their item, but it's also your hard earned cash and if they do not wish to do business then it's not worth whining about. Above else, be polite at all times. Like it or not whether you are online or on an overseas trip you are an ambesseduer of your country. Your words and actions shapes the image of what others think of your countrymen.
Best regards
Azad