I'm interested that a number of people have stated that sellers in the US are more confident of getting a domestic sale as the USA is a larger country. The USA has a population of just over 300 million while the EU has a population of just over 500 million. Is there really so much less interest in fountain pens and other writing instruments per head of population in the EU than the USA? I understand that there are other barriers such as inflated shipping costs and customs documents to fill out which the seller may feel would discourage a sale or cause them personal inconvenience but I've shipped items all over the world using couriers from the UK and have found it relatively easy to do and very reliable. There are certain countries I'd be wary of shipping to but the EU as a whole is painless.
I think it's just that many users on FPN are located in the US and expect that members will be located nearby too. Unless the pen is uncommon many people are happy to be trading between states even though they can expand their market if they include EU too. I admit FP collecting is seen as a bit of a novelty here in the US. Also, Paypal requires sellers to use Priority International which usually means +$24 on even the smallest packages.
I usually allow international sales but I like to remind buyers that it'll be First Class Mail which is untracked. I have bought items from different countries and I'm aware of the large range of shipping times.