You can allow your users to preview themes without activating them, this way you can setup a website providing different downloadable themes to your users and easily allow them to preview each theme.
Install and activate WordPress Theme Demo Bar, it allows any WP theme to be previewed without activation by using a demo bar at the top of your website, it allows your users to preview another theme easily. Bar is customizable from admin area dashboard and more than 1 extra CSS files can be uploaded.
After activation visit its settings page and set the options. Its quite simple to use, you have to add themedemo variable with your website‘s address followed by the theme name you want to preview, it would become somewhat like following URL:
http://www.yoursite.com/?themedemo=theme-name
Now this URL would be showing yoursite.com with Demo Bar showing theme switcher, you can move back using the ‘close’ button. It automatically picks themes from your themes directory and by using theme switcher drop down box one can preview your themes (including child themes). More of its features are discussed on its download page.