You should be able to use it from the US. Just choose currency as Euros, select Amazon vouchers in a few European countries as payment, then whatever sell offer you get, go to their country's Amazon site with your normal account to buy the voucher and pay in Euros. Easy peasy.

For some reason (guessing legal) it only allows bank transfer if I select GBP but I hit EUR and I can add Amazon gift cards from all over the EU.

Nothing fishy about it from Amazon's end. You just had to buy a voucher for a friend abroad. I didn't even get extra verification when I went to pay or anything.