In theory I agree with Chip's assertion above that "a Theme shouldn’t *need* post-installation support. It should just work as-designed."

Unfortunately a great many people who purchase themes don't really understand how WordPress itself works and then tend to blame the theme for their own inexperience.

For example many new-to-WordPress folks think they are buying a "website" when they purchase a theme and therefore want to create an exact duplicate of the theme demo as their starting point. But when the WP XML import doesn't import the demo's widgets their first thought is "the theme doesn't work."

Without solid post-purchase support a shop would quickly get a horrible (and unfair) negative reputation as some of these folks can get pretty noisy complaining out of ignorance no matter how well their themes are coded. Therefore post-purchase support is a cost of offering themes and needs to be factored into theme pricing in order for a theme shop to be successful.