How can you showcase premium themes to clients without having to pre-purchase the theme?

One option would be to display screenshots of the theme in action. Many theme services allow you to view a “demo page” and see how the theme is presented – taking a screenshot of this page and passing it to your client is pretty much the same as presenting a stylized wireframe or stylistic mockup of the final design. If it was an entirely custom site (i.e. you were responsible for the design as well), you’d likely present it this way anyway before investing any time in actual development.