You can buy one of those for $5 but I can almost guarantee we will see you posting soon enough in tech support about how your cd-key is banned and how you never did anything wrong.
Just my .02
If you buy a key other than Blizzard, you will most likely either:
1.) Receive one that has been banned or muted
2.) Receive one that has been sold already to several people
3.) Receive an illegal one that will result in a 2 week IP ban when you attempt to connect with it.