1.) Go to serials.ws or youserials.com and find a temporary cd key.
2.) This is where your almighty Photoshopping skills come in, use the temporary cd key you found and Photoshop it onto bottom of booklet in the following image: (CLICK HERE). Email me if you need assistance with Photoshopping at email@example.com.
3.) Make sure you have an EA account, if not, you can make one HERE.
Now here comes the fun part!
4.) Go to the EA Chat Support HERE and log into your EA account. Wait in line for your rep, and be patient.
5.) When it is your turn, say "Hello, I bought Bad Company 2 from a game shop and whenever I tried to input the cd key it says it has been activated already." They might tell you to go to the shop and solve the problem with them, but just say "They told me to contact EA for support." Also, they might ask for a receipt, tell them "Can you give me a few minutes to look for it?" Then reply in 2-5 minutes saying you can't find the receipt, but say I can take a picture of the cd key booklet (which has already been Photoshopped).
6.) After you send the rep the photo, that is when they will most likely send you back another. When you retrieve it, be sure to tell them "Thank you!" and be friendly with them.
7.) Now that you have the cd key, download and install the EA Download Manager HERE.
8.) Run the EADM and log into that and hit the Tools icon on the upper right corner and redeem your cd key.
9.) Install the game and enjoy it!
10.) And I hope you enjoyed this wonderful method.