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.
You can play Star Wars Battlefront II over LAN regardless of any registration keys.
This holds for the retail/CD/DVD version of the game and not the Steam/GoG version (which requires a valid copy of the game for each player to start it in the first place). There is no key check performed for LAN games, so technically you can even use the same copy of the game twice (by copying the game files to the other computer).
As the original online multiplayer does not work anymore and most alternatives do not perform any registration key verification you can safely buy the game anywhere without worrying much about the registration key. (I am unsure whether SWBFSpy - a online multiplayer alternative - does perform key verifications or not; however even if it does, the chance is negligible that if you get an already-used key someone is playing with it at the very same moment you want to play.)
However, if you are looking into buying the game, I recommend buying on Steam or GoG, as you will have access to the re-enabled Steam/GoG-only online multiplayer as well as having the opportunity to use other alternatives like SWBFSpy.
