Creating Sign Up form for 2 types users in Drupal 7
Sounds like a way to go. Additionally you may use the Rules module to redirect the seller to the profile page and the LoginToboggan module to login users immediately after registration. Also some Spam bot prevention may be useful. Generally I consider registration without user approval (by email) as harmful: A user can easily register himself in the name of another person. Drupals double opt-in usually helps to prevent that. LoginToboggan can delete users which did not approve their registration after x days.

Anyway here is a potentially cooking list:


Add two roles: Buyer and Seller.
Use the Auto Assign Role module to let the user choose between Buyer and Seller at registration time.
Use the LoginToboggan module to login the user immediately after registration.
Create a profile for Sellers using the Profile 2 module. Use Drupal permissions to make it accessible by Sellers only.
Use the Rules module to redirect Sellers to their profile page if they have no profile yet.
Optionally use the CAPTCHA module to prevent spam bot registrations.