You'll want to buy cd keys from local stores if possible. Online keys can be warez and there's no way to know if the key is legit or not. If you have to buy online, try using a reputable site like ebay or amazon.
Buying these keys should only be done if offered by the distibutor of the software.
Say you play World of Warcraft (like I do) and you want the Wrath of the Lich King expansion, you can get it online at the Blizzard (developers of the game) site only: http://www.worldofwarcraft.com/accou...e-upgrade.html .
Any other site who offers keys online I wouldnt trust.
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