If i were to buy a used copy of Halo CE for PC off of ebay, would i get an invalid CD key message?
Halo PC CD keys do not have a use limit and you can use a second hand copy to install and play the game, including online multiplayer. The only issue you may encounter would be whether the seller keeps a copy of the CD key and uses it. If they do, then you will still be able to play online unless they join the exact same server. If two people are using the same CD key in the same server, the player trying to join will not be able to connect and you will get the invalid CD key message. That said, given the vast amount of servers, it’s highly unlikely the seller will continue to use the CD key and it’s highly unlikely you’ll end up in the same server anyway.
As a side note, only Halo 2 Vista has a CD key install limit, where the CD key can only be used 5 times to install the game before requiring a brand new CD key.
Hope this helps.