It's tougher than the traditional form of selling. Being an online sales person, I found it interesting and very comprehensive process of learning. Sale is always a combine activity of good product/service knowledge and sharp presence of mind. If you know the rule, you can sell anything.
Definition: May be you can Google it.
Let's proceed with some funny simple ideas:
# Read it: A through knowledge about the product/services is most important. How can you sell a product which you don't even know? You must know the product before you go to sales it. This will give you an idea about your customer/client.
# Listen: The quieter you become the more you are able to hear. Listen to your perspective, understand the requirements. However, it doesn't always necessary to allow your prospective speak more. [will come to it later]
# Address: Try to address the prospective's queries if that belongs to both of your interest. Be careful, non-sense is not allowed!
# Match the requirements: Be sure, if you're not able to match your product with perspective's requirements, you can't win. Here the product knowledge helps to get the sales done.
Sale is never the end process, but a new relation.