The games SHOULD come sealed if you bought it new. If it came opened, I'd file a complaint about it.

I haven't bought a new game from EB Games/GameStop online in some time, but every time I have in the past, it's always arrived sealed.

I have heard rumors that employees of these stores sometimes open games, probably for putting out on display. The game disc is kept in a secure location, while the case is put out on display for people to look at. It is at least possible that's what happened in your case, though I'd still file a complaint.

If the disc looks decidedly used, I would just take it to the nearest EB Games/GameStop store if one is handy, along with a copy of your online order receipt, and ask them to replace it. Most companies are pretty willing to replace/exchange items, it's when you start looking for refunds they tend to balk.