What if I already have the game?

I already have Prophecies. I wanted to purchase Nightfall, and I found just the code for sale. How would it work to just use the code?

Start the Game client, you then have the option to enter a new key and your all set. Add it to your existent account and you will be able to play it all together.

If somebody is just selling the code and they're not Anet, unless they're a close friend I wouldn't bother. The individual campaigns are fairly cheap as it stands in your local games shop so why risk buying dodgy goods?

on character select screen, go to edit account details and enter the new code.
enjoy.

You do need to be careful, one of the more common scams is to buy the code online with a stolen card, resale the code. Another scam is to protest the origional sale after you re buy the code. Another is to use the origional proof of purchase to reclaim the account claiming you stole it.

In Guild Wars; pocession is 100% of the law, the game will always belong the the holder of the origional game code. If you are rebuying the game make sure you get the origional code, not a photocopy, not an email, the origonal.