sell your product or services to a complete unknown or to an individual you've had contact previously

Let me ask you a question: do you believe it's simpler to sell your product or services to a complete unknown or to an individual you've had contact previously; whether it's an e-mail conversation, forum participation or even an existing buyer relationship (i.e. He's purchased something from you).
Beyond any doubt, it will be more difficult to sell to a total unknown who doesn't know whether or not you're an honest seller, and whether or not they ought to trust your words and trust that your product or services will really benefit them.
This is exactly why you ought to construct a buyer base -- a group of individuals who trust you and will most likely buy a product that you provide to them. This will ensure that you've a recurring and consistent source of revenue. Once you release a product or make an endorsement, you've a group of individuals always ready to hear what you have to state