I'm an eBay seller, and believe me, there are more problems than you can believe arising from crappy international shipping services.
1. There is no way of knowing before hand what kind of fees you'll be charged. The best you can do, is call your post office and make them give you an estimate. Before you do this, you'll need to know the approximate package size/weight, the shipping service you intend to use, and (of course) the country of origin. Remember, ALL fees will be charged to you by your country's shipping service, NOT the eBay seller. The eBay seller only needs the money required to ship the package to your country. All other fees will come in on your end when you go to the post office to pick it up.
2. DO NOT SHIP WITH USPS (United States Postal Service). I've shipped three large packages internationally with USPS. Two of them took over a month to arrive, and the other failed to leave the country, even after two attempts. This ended up costing me $250 and the loss of a sale. The others did get there eventually, but the time it took was ridiculous. I am NEVER using them again for International Shipping. Use UPS, FedEx or something like that. It will cost a good deal more, but at least you will get your package.
Good luck/hope that helps.