How can I build a wordpress theme which I could sale on themeforest type websites?

Developing an wordpress theme is very easy you just have to follow some wordpress-oriented recommendations. The easiset way to develop a wordpress theme is to download a copy of wordpress and go wp-admin->themes folder, there you'll see pre-installed theme. Open the theme of your choice in a text-editor and change the CSS according to your need. Hope this answers the question.

Complete video tutorials for learning Wordpress.

Nice tutorials.

Please visit this URL:

http://openvideotutorial.com/computer%20science/Wordpress%20Theme%20Tutorial/Wordpress%20Theme%20Tutorial1.html

Try artisteer software

http://www.artisteer.com