> How can one verify that a vendor has been licensed to sell a work if they don't include their license with the author?
One generally is not expected to, unless one is the author or author's representative, in which case one doesn't need the vendor to provide the license agreement.
> Maybe a street vendor has been licensed to sell the just-appeared-in-theater movie that they are selling?
A more valid analogy here would be "maybe the multiplex cinema selling movie tickets has been licensed to screen the just-appeared-in-theater movie that they are showing".
One generally is not expected to, unless one is the author or author's representative, in which case one doesn't need the vendor to provide the license agreement.
> Maybe a street vendor has been licensed to sell the just-appeared-in-theater movie that they are selling?
A more valid analogy here would be "maybe the multiplex cinema selling movie tickets has been licensed to screen the just-appeared-in-theater movie that they are showing".