If you sell on eBay, the likelihood is that you will have come across a buyer from a different country. eBay offers a 'Global Shipping Programme' (GSP), this works by having a UK distribution centre that sends packages to overseas postal addresses. eBay offers this service to all sellers, and you can opt out of the programme at a listing, country or account level at any time. You will have received an email from eBay if you are eligible for their GSP.
eBay notify sellers when listings are being enrolled into the GSP. When listings are enrolled in GSP, international postal charges are automatically shown on your listings and paid in advance by the buyer.
When you sell an item that is enrolled with GSP you simply post it to the UK Shipping Centre, this address is made the default location for the postage. When posting the package, you will need to attach the unique identifier code because this is how the UK Shipping Centre identifies your item.
Once your item arrives at the UK Shipping Centre, the item is sent to the buyer via a fully-tracked international postage service.
Returns are not handled through the GSP. If you need to give a refund on an item then you will only need to refund the domestic postage and the item cost, eBay will cover the international postage costs. If the buyer needs to return the item, you will need to resolve the postage costs and possible refunds directly with them, dependant on the reason for return.
You can read all about this feature here: http://portal.ebay.co.uk/gsp