I actually work at an EB Games and nothing makes me cringe more than putting those stupid stickers on the unsealed boxes. Taking them off is nearly impossible. However, people looking specifically for factory sealed copies are the exception rather then the rule, at least from what I've seen.
As for the demos question, unfortunately no, we do not get to demo new releases. Although I really wish we could because at least then I could provide some credible first hand experience for alot more of the games. Minimum wage does not really allow me to buy up all the new titles.
And the open boxes, yeah that sucks too, but like I said, alot of people couldn't care less about the game being opened. They should be sold at a reduced price, but when people are willing to pay full price well, thats economics for you I guess. I don't agree with it but unfortunately thats the way it is. Usually when someone comes in looking for a sealed copy of a game and we've only got a gutted one I'll phone the other four stores in the city. I'm not sure if other EB's/Gamestop's will do that.