I mean that's a pretty big dent in the box; My iMac box, even with the sturdy outer box, had a tiny tiny hole in it, but I wasn't that bothered about it cause it was like just a bit bigger in length than a fingernail and not very wide either. It gave me enough concern to carefully look over the computer after taking it out, but finding no issue cosmetically or functionally - other than software issues of which I still have more than I'd like - all of the concerns were put to rest and it's not really something that's on my mind any longer.
Are you buying these from Apple/Apple Premium Resellers?
My advice would be like Allan Nyholm; Contact the seller about it to have it be documented that this was the case - you also have these images and this post to reference and make sure they know about It in advance. - Get them to sort of say that you can open and inspect the computer and still not be liable if you find issues with it after opening it, and if they seem fine with you still being able to return it even if you open it, start using it and see if it's fine inside.