This is from a while back and relates to the US, not Europe, but it's an interesting read:
http://www.promenademusic.co.uk/importing-guitars-from-america
As far as bringing something in and claiming you already owned it and took it out with you, it's a strategy that could work, but there is a risk. As Chrisb states above, it becomes your problem to provide proof of ownership.
However, as this is about deals in Europe, for the short term at least, there's no problem.