I really dont know where to post this but I have some general questions.
Can we buy a template from here and modify it and sell it to our clients?
Can we buy stuff from flashden (like gallery) and use them in our website which is going to be in sale in themeforest?
You can modify templates purchased here at ThemeForest and sell it again to your client. However, for each client, you will need to purchase the template. 5 clients, 5 purchases. You are essentially purchasing a license to use it.
You can purchase items from flashden and use it within a template which can then be sold to a client. Again, 5 clients 5 purchases (of both html template and flash item).
If you want to post a template for sale on ThemeForest, the template must be your own work or if you used pieces of someone elses work, you need to get permission first.
Hope this helps!