CD Kry for PC Game Already Used

I purchased a PC game that came in an open box. they brought the disc and booklet containing the activation code seperately to the counter, i inspected the disc but did not think to question the activation code. At home i proceeded to install the game and entered the activation code only to be met with a screen saying that this code has already been activated and and is assigned to a different user. Clearly the staff either used the code themselves or passed it onto a third party. I now sit with a piece of software that i cannot use and an activation code that has been ****** from me! This is unacceptable and i will never purchase software from your stores again.