buying a used copy Gal Civ 2

Yes it is.
Not only that, the license is not transferable from one person to another.

And by the way, the serial number is not located on the back of the box.
It is on a sticker affixed to the cd case.

The guy selling it is scamming, and you would be wasting your money.

Yes. Any buyer will not be able to register or use any patches without obtaining their own serial.

There's also a more than fair chance that the game is a pirated copy (of the three auctions I just found with a quick search, one is quite clearly an illegal copy), or the seller already registered the serial and plans to coninue playing after selling the disc (and is just making an excuse).

Thanks a lot. It sounded a little strange to me as well. Think I'll just buy a new copy.