In the prologue, I can't leave the bookstore. The robe shop has an 'Exit' and the other shops have you leave when you choose the "I'll come back later" option in dialogue. I can't find anything to select other than Hermoine or the store clerk, and while the clerk has an "I'll come back later" option, it doesn't make you leave the store.

Edit: I should add this is on 0.91b PC version.