You buy it new in box but the key is already used?

Sorry to hear that. That's crazy!

Its possible that someone that works at whatever store you bought it from did a nifty job of opening it up, and using the code, then close it back. I've had this happen numerous times especially when I buy online.

Now, I only buy games like this straight from the company that makes it or buy a hard copy from a reputable local store like gamestop where this is much less likely to happen.

Where did you buy it from?