I've dabbled with companies like Envato/Themeforest,Mojo themes, etc. and have had clients come to me with themes from them as well. By and large the experience has been very poor. Oddly coded and configured themes that have very poor documentation and sketchy support.

Unless I am developing my own theme I only use and recommend themes that are in the WP repository. These have been vetted by WP and are of good quality.