Should the camera store take it back? Yes, if you feel it is defective, I can't imagine any reputable dealer not accepting the return.
Is it defective? Honestly, as others are saying, it's pretty hard to tell from your examples, but it looks like you are showing us crops that represent 5% or less of the total frame. Have you compared these images to similar crops from your previous camera? Is it less sharp than what you had?
This reminds me of a thread awhile back from someone who felt his 7D wasn't performing up to par and no one else could see any problem. Bottom line is, if you aren't satisfied with it, there is probably nothing that people on this forum can say to change your mind so you may be better off returning it and either getting a different body or reconsidering if this is the camera for you.