Wordpress theme shops either use a theme switcher plugin (only displays frontpage usually) or a Wordpress Multi-Site installation (so all aspects of the theme can be demonstrated) combined with iframes so it can be shown under one domain...

This can be reconfigured in Ghost..

1. Multiple ghost installations for each theme. This means subdomains.
demo1.mydomain.com
demo2.mydomain.com... etc.

2. Create a custom page theme that displays fullwidth posts and use iframes...

See demo here. (I tested to see if the proof of concept works)
http://leoadhemartan.com/test-iframe-concept-demo-site-ghost-themes/