What could be the possible questions that the detective could have asked to Sam, the grocer?

I think that the first answer has most of the possible
questions.  I think that the questions about Mary's state of mind are the most
important.

Because we have read the story, we know that
Mary has rehearsed what she is going to say to Sam.  We know that she is going to try
hard to make him notice her and remember that she was there at that particular instant. 
You would think that this would make her act in an unnatural
way.

So I think the detective really should ask Sam about
how Mary acted.  He should ask if she seemed like herself.  He might even push and ask
specific questions -- did she seem excited, did she seem like she was trying to make you
remember her, did she seem like she was playing a rehearsed
role?

I think that the question about what she was carrying
is a little less useful -- after all, I doubt she would go to the store carrying the
murder weapon.