By buying leads month after a month, a network marketing rep builds his or her own list of leads, usually in an autoresponder. You can then continue to market to these people for years to come. The best idea to continually give them value, your emails could perhaps have a ratio of 80% value to 20% sales. If all you do is send sales offers to your list, you will see your open rates go down, and your unsubscribe rates go up.
The reason I started Apache Leads way back in 2003 was that I was a Diamond level distributor for a San Diego based MLM company and had quite a large business. None of my associates had any access to leads, so that's how we got started. The demand for leads grew so fast that I had to focus on getting quality, affordable leads in ever increasing numbers.