I have created multiple themes, but I don't know how I can demo them on my web site using wordpress blogs. How do other sites do it to demo their themes?
I think they use third party website, vendors and services which allow live demos of the template the web visitor may try to purchase. Do this, when you see a site with a live demo look at the url. I think it's template-access.com I could be off
Use the theme switcher plugin - this way you can have all your themes demos on just one Wordpress installation.