She has no shoes becuase right before she goes into the shop, she takes them off. I think she was wearing heels and she didn't wanna trip and fall or something.
Actually in the real movie. Sarah Michelle Gellar ran too fast for the killer so they made her wear high heels. I think that they fell off when she jumped out the window.
Yes I believe you see her take them off. But wouldn't you do that if you had to run from a killer? I think thats more of a trying-to-save-her-life-thing than a slip up ;) I heard that in Scream she had to wear heals in her death scene because she is such as speedy runner!