If you buy this game 2nd hand you need to find out a few details:
If the player never registered for a Stardock.net account, the Metaverse, or downloaded any patches through Stardock Central or even standalones, then you're fine, the serial number isn't associated anywhere with personal ID.
However, if the original buyer did any of those things then you need to get the email address he used and password if applicable. In this case you need to change the user information over to you with those two pieces of info.
If the original buyer has other Stardock software and he registered GalCiv, forget about buying it because you won't be able to register it at all.