I have purchased 5 templates from themeforest. They have worked very well! It helps to read the customer reviews and ratings when buying a template. I have also used their sister site codecanyon.net to buy wordpress plugins and graphicriver.net for stock images.

http://themeforest.net http://graphicriver.net http://codecanyon.net

Some of my favorite wordpress themes form themeforest: http://themeforest.net/item/pandora-wordpress-business-portf... http://themeforest.net/item/bigfeature-wordpress-theme/51702 This one lets you sell digital downloads: http://themeforest.net/item/the-furniture-store-wordpress-ec...