Buying Star Wars Battlefront II for PC safely

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.