Depends on who you ask (no if you ask Sky!). There is a legal battle going on in the UK courts at the moment with regard to foriegn decoder boxes being used there. Part of the premise is that there is a European directive that says that that they are legal and UK one saying there not (typical). Also Sky have only licenced the material they show for the UK and Ireland. For example Canal+ have similar rights licences in Spain.
So even if the European directive is found to be over-riding and there is no restriction on buy and selling boxes anywhere in Europe, You’d find that in Sky’s T&C;‘s they still prohibit use of the card outside the UK and Ireland, as like a bank card it remains their property and you only have a right to use it.
Short answer then is don’t tell them!
