Selling themes doesn't seem to infringe on the community at all.

In this case, it's good that "buying out" a theme is an option because some clients can afford it and they want a distinct look which not everyone else will have.

Themes are more like art than software. It doesn't bother me at all to have themes which are available to be licensed or bought.

Modules are another story. I don't want Drupal's module selection to become commercialized like Joomla, i.e. for sale.

Support licenses or contracts for Drupal are another thing entirely because it is a service. I would recommend clients buy support licenses for Drupal if their budget and their needs make it worthwhile.

I expect to develop a lot more commercial sites with Drupal. I've also got my eye on Sovix for community projects which don't require as much privacy.