While it seems you may be more aware, over cautious, etc than most users, especially since the post seems like you received two cameras in one day and have issues with both?
Either way I understand where your coming from, i got a 17-555 2.8 once and when i got to my car and opened it i had a bad feeling, mounted it and shot a few, felt like it was taken from the packaging, which is not uncommon at local stores, they open one and let you test it, then re box it. I took it back they guy inspected it and said it wasn't sold but may have been used int he store, i said for the price i paid take one from the bottom of the pile (which was ont the shelf i could see) and while it wasn't any different i had a better feeling.
Its your money, its a lot of money to most and I would suggest you may want to purchase your next one at a local auth cannon dealer allowing you to inspect before leaving.
The $7 res-otck is better than most 15% interent retailers charge these days.
Hopefully a good copy falls your way shortly.