Razlaw,
First of all, many dealers in the US do trades. I take trades constantly and it helps me to satisfy a broader range of clients.
Secondly, the BeoVision 7-55 is not a BeoSystem 3. It has the components of one built in, but it's the TV. I'd have to research how to make it work, but using the BV12 as your second screen would be a pain in the ass (assuming you could even hook it up and get it to operate, which I have doubts about) in so far as the BV12 would always be the Cinema Mode (Format 0).
Find a dealer to make the trade is my suggestion.