It's so nice to have many theming sites. Provided they are Drupal themer sites:
Although some of their demos have the "Mambo license" menu item running, which is quite frankly not a testament to their understanding of Drupal. However, starting off from a ported theme could still be nice, those buying Drupal themes might not want to fiddle as much customizing the theme further.
However, starting off from a ported theme, with invalid xHTML and almost none of the detail will give you frustration and the customer service would be better off noneixsting because then it'd be at least clear what's on. There is one thing to do with TemplateMonster: avoid. Support those who work with Drupal, for Drupal!