In business, the easiest customers to market to are the ones you already have. They have already proven an interest in what you are selling. They already know about your products, services, and order processes. Confirmation bias leads them to prefer you to any new market entrant.

Net Builder slaydragon has written a couple of useful blog posts applying this general business principle to the web:

Most of us spend a lot of time and money on SEO (Search Engine Optimization) to bring new visitors to our web sites. How much more money could we make if we devoted time and effort into getting those visitors to come back?