We have several demo sites for each theme which allow you to preview the design and functionality. There are multiple demo shops for a theme, which will display different options and features available to enable from the Theme Settings.
The Shopify Theme store allows you install our themes in a 'theme trial', which allows you to completely setup the theme 'unpublished' without affecting your current shop. The only limitation is that you do not have access to edit the template files directly until the theme is purchased.
When you're ready to purchase, the theme can then be published as your main theme. In order to purchase the theme trial, in the Theme library in your Shopify Admin, click Actions > Buy.