Probably it was a technical problem that was solved soon enough. I had contacted the seller before, and she accepted international offers. I usually wait the seller to send me an invoice with the exact shipping cost, when it is not clear at the auction for my country. As far as I remember, when I push ''Buy it now'', first I see a confirmation message that I bought this item and then I proceeded to the order confirmation and payment page. And then I asked for an invoice if I needed to. In this case, I did not saw this confirmation message, but was straight directed to the payment ebay page. And there, I could not do anything. After several tries I managed to pay, putting the shipping cost myself.
As I wrote, maybe it was a technical problem, as I saw at the I saw at the announcement section that sellers were facing a problem when trying to viewing the order details page too.
I still cannot undertstand though, why I have not received the usual ebay message to my email ''You've won this item''....
Edit: The listing had only ''Buy it now'' option.