To be perfectly frank, I find all this to be a moot point.
We exist in a global economy selling items of one country's origins in numerous countries (by this I mean for example a US 1 sheet CAN be found for sale in the US, UK and various other countries), the items of which are in multiple supply. We none of us are existing in a single country's economy to the exclusion of any other country.
For me, this comes within the same parentheses as BPs. To wit; a buyer will have a budget to get A poster from Seller to Home. How that budget is spent can have multiple avenues which will include, but not be limited to, BP, Tax, Customs, Shipping, Exchange Rate, and other stuff. The best case scenario being, you live next door the seller which means all the budget can be spent on the poster; the worst case meaning all of the aforementioned factors eating into the deal.
It all works itself out in the end.