How Do You Handle Bad Manners?

At my weekend yard sale, a girl about 10 years old stopped by.  She was by herself, no parents around.  She seemed interested in a game that I had for sale so I said, "You can just have that.  No charge."  She walked away with the game, never saying, 'thanks."  And that bugged me.

True, I essentially gave her a gift; and gifts are not supposed to come with conditions, right?  Was it fair of me to expect her to thank me?  But then I thought, it's just common courtesy that when someone gives you something, you are supposed to thank them.

I sooooo wanted to say something like, "You know when someone gives you something, the polite thing to do is to say thanks."  But then I thought it's not my place to reprimand another woman's child.  But would it have been so wrong for me to say something in an effort to educate her on social norms?

I'm really interested in your opinion on this so please post a comment or two.