Why not just simply create a test site with WordPress multisite where potential buyers can test drive your themes?
A multisite installation is also a good idea… but correct me if I’m wrong. You, as admin of the network, would need to create each theme installation manually for each user asking for a demo, right?
Wrong, you’ll only need to put the themes under wp-content and allow user registration in the WP multisite instalation. Each user will then have a new blog after he/she registered and can pick any themes they like to test.