You have missed the point. The collectable may become available because of the GSP because the seller doesn't know the item is being offered outside the US/UK. As far as they know, they are only offering domestic post. They don't know that they have been automatically opted into the GSP. Often the first they know about it is when someone like yourself contacts them asking them to change postage method. Therefore, because they have been opted in to the GSP, you are able to buy it. If they had previously opted out and only post within their own country, then you won't be able to buy it. So yes, items can become available because of the GSP.