As our company title suggests "AppThemes", we specialize in application themes for WordPress rather than plugins ;) We can certainly understand that people may like to see our products as plugins, however having these as themes allows us to really make the most of WordPress and thus offer more features and functions than we could achieve with the use of a plugin. There's ultimately absolutely no reason why you can't use our themes with an existing website if you consider installing them in a subdirectory and restyle the theme to "match" your homepage.
e.g.
yourdomain.com <== Your existing website
yourdomain.com/classifieds <== Your ClassiPress installation
yourdomain.com/jobs <== Your JobRoller installation
Then to tie it all in, you could set this up on a
WordPress multisite network to allow your customers one login across all installations, giving it a seamless browsing experience.
We appreciate your feedback :)