B&W shows right on their web site:
"2. The warranty is only valid for the original owner. It is not transferable."
Even if you are the first person to "register" those speakers, if you ever need warranty service they are likely going to ask for the original proof of purchase, which should have the original owner's name somewhere on it. If it does not, then you would probably be ok. The dealer may be wanting to sell a new set of speakers to the original owner of these B&Ws, so might be giving you the wrong information about warranty transferability.