Buying Star Wars Battlefront II for PC safely

I am considering getting another copy of Star Wars Battlefront II so that I and a friend can play together over LAN. I am looking on E-bay and Amazon and am concerned about the registration key that was given with the copy I have now.

If I get a copy that does not have manual (and thus no registration code), will the one that I have now work? Or do I need to make sure that I get a copy of Battlefront II that comes with the manual and the appropriate key?

You will typically need the additional key. If a game is registered with a service like Steam, you cannot use the same key for multiple accounts. Some games which don't use such a registration service will instead prevent players with the same key from playing together. It is important that when purchasing a game, even if they include a separate key, it may already have been used, rendering it useless.

I recommend going through a trusted retailer for your purchases to prevent this issue. This could be physical stores or online retailers such as Steam or Good Old Games.

In particular Steam currently has Battlefront II on sale for less than $3, depending on your currency, see here.