I've searched the forums here, on STEAM and the .com to no help so far

I have purchased a 2nd hand boxed DVD copy of Empire. However, as it was pre-registered with STEAM it won't allow me to install with a new STEAM account.

How do I get round this? I'm going to assume that if I'd chosen "register with existing account" for installation it would have asked me for log-in details I just do not have.

Anyone else encountered this issue? Should it even BE an issue? Is this preventing follow-on sales by legit methods (i.e. trade-in stores etc)?